diff --git a/addons/hr_timesheet/report/project_report_view.xml b/addons/hr_timesheet/report/project_report_view.xml
index 698c22704a7..fe3fdaf6a76 100644
--- a/addons/hr_timesheet/report/project_report_view.xml
+++ b/addons/hr_timesheet/report/project_report_view.xml
@@ -39,7 +39,7 @@
                     <filter string="Tasks in Overtime" name="overtime" domain="[('task_id.overtime', '&gt;', 0)]"/>
                 </xpath>
                 <xpath expr="//filter[@name='my_tasks']" position="before">
-                    <filter string="My Team's Projects" name="my_team_projects"  domain="[('project_id.user_id.employee_id.parent_id.user_id', '=', uid), ('project_id.user_id', '!=', uid), ('user_id', '!=', uid), ('user_id', '!=', False)]"/>
+                    <filter string="My Team's Projects" name="my_team_projects"  domain="[('project_id.user_id.employee_id.parent_id.user_id', '=', uid), ('project_id.user_id', '!=', uid), ('user_ids', '!=', uid), ('user_ids', '!=', False)]"/>
                 </xpath>
                 <xpath expr="//filter[@name='my_tasks']" position="after">
                     <filter string="My Team's Tasks" name="my_team_tasks" domain="[('task_id.user_ids.employee_id.parent_id.user_id', '=', uid)]" />
@@ -52,7 +52,7 @@
             <field name="model">report.project.task.user</field>
             <field name="inherit_id" ref="project.report_project_task_user_view_tree"/>
             <field name="arch" type="xml">
-                <field name="user_id" position="after">
+                <field name="user_ids" position="after">
                     <field name="hours_effective" optional="hide" widget="float_time"/>
                     <field name="progress" optional="hide" widget="progressbar"/>
                 </field>
diff --git a/addons/project/report/project_report.py b/addons/project/report/project_report.py
index 080e6fa66fb..bddd5bd47cc 100644
--- a/addons/project/report/project_report.py
+++ b/addons/project/report/project_report.py
@@ -11,7 +11,8 @@ class ReportProjectTaskUser(models.Model):
     _auto = False
 
     name = fields.Char(string='Task', readonly=True)
-    user_id = fields.Many2one('res.users', string='Assigned To', readonly=True)
+    user_ids = fields.Many2many('res.users', relation='project_task_user_rel', column1='task_id', column2='user_id',
+                                string='Assignees', readonly=True)
     create_date = fields.Datetime("Create Date", readonly=True)
     date_assign = fields.Datetime(string='Assignment Date', readonly=True)
     date_end = fields.Datetime(string='Ending Date', readonly=True)
@@ -55,7 +56,6 @@ class ReportProjectTaskUser(models.Model):
                     t.date_end as date_end,
                     t.date_last_stage_update as date_last_stage_update,
                     t.date_deadline as date_deadline,
-                    tu.user_id,
                     t.project_id,
                     t.priority,
                     t.name as name,
@@ -82,7 +82,6 @@ class ReportProjectTaskUser(models.Model):
                     t.date_end,
                     t.date_deadline,
                     t.date_last_stage_update,
-                    tu.user_id,
                     t.project_id,
                     t.priority,
                     t.name,
diff --git a/addons/project/report/project_report_views.xml b/addons/project/report/project_report_views.xml
index c44b0e9d95a..1f01e7f067c 100644
--- a/addons/project/report/project_report_views.xml
+++ b/addons/project/report/project_report_views.xml
@@ -17,7 +17,7 @@
             <field name="arch" type="xml">
                 <graph string="Tasks Analysis" sample="1">
                      <field name="project_id"/>
-                     <field name="user_id"/>
+                     <field name="user_ids"/>
                      <field name="nbr" invisible="1"/>
                  </graph>
              </field>
@@ -31,7 +31,7 @@
                     <field name="name"/>
                     <field name="partner_id" optional="hide"/>
                     <field name="project_id" optional="show"/>
-                    <field name="user_id" optional="show" widget="many2one_avatar_user"/>
+                    <field name="user_ids" optional="show" widget="many2many_avatar_user"/>
                     <field name="stage_id" optional="show"/>
                     <field name="company_id" optional="show" groups="base.group_multi_company"/>
                 </tree>
@@ -46,7 +46,7 @@
                     <field name="project_id"/>
                     <field name="name" />
                     <field name="stage_id"/>
-                    <field name="user_id"/>
+                    <field name="user_ids"/>
                     <field name="partner_id" filter_domain="[('partner_id', 'child_of', self)]"/>
                     <field name="name" string="Tags" filter_domain="[('task_id.tag_ids', 'ilike', self)]"/>
                     <field name="date_assign"/>
@@ -59,7 +59,7 @@
                     <filter string="Starred" name="starred" domain="[('task_id.priority', 'in', [0, 1])]"/>
                     <separator/>
                     <filter string="Tasks Late" name="late" domain="[('task_id.date_deadline', '&lt;', context_today().strftime('%Y-%m-%d'))]"/>
-                    <filter string="Unassigned Tasks" name="unassigned" domain="[('user_id', '=', False)]"/>
+                    <filter string="Unassigned Tasks" name="unassigned" domain="[('user_ids', '=', False)]"/>
                     <filter string="Open tasks" name="open_tasks" domain="[('stage_id.fold', '=', False), ('stage_id.is_closed', '=', False)]"/>
                     <separator/>
                     <filter name="filter_date_deadline" date="date_deadline"/>
@@ -72,7 +72,7 @@
                     <group expand="1" string="Group By">
                         <filter string="Project" name="project" context="{'group_by': 'project_id'}"/>
                         <filter string="Stage" name="Stage" context="{'group_by': 'stage_id'}"/>
-                        <filter string="Assigned to" name="User" context="{'group_by': 'user_id'}"/>
+                        <filter string="Assigned to" name="User" context="{'group_by': 'user_ids'}"/>
                         <filter string="Customer" name="Customer" context="{'group_by': 'partner_id'}"/>
                         <filter string="Deadline" name="deadline" context="{'group_by': 'date_deadline'}"/>
                     </group>
diff --git a/addons/project/report/project_task_burndown_chart_report.py b/addons/project/report/project_task_burndown_chart_report.py
index ecc5ced3258..6850267e304 100644
--- a/addons/project/report/project_task_burndown_chart_report.py
+++ b/addons/project/report/project_task_burndown_chart_report.py
@@ -18,7 +18,8 @@ class ReportProjectTaskBurndownChart(models.Model):
     display_project_id = fields.Many2one('project.project', readonly=True)
     stage_id = fields.Many2one('project.task.type', readonly=True)
     date = fields.Datetime('Date', readonly=True)
-    user_id = fields.Many2one('res.users', string='Assigned to', readonly=True)
+    user_ids = fields.Many2many('res.users', relation='project_task_user_rel', column1='task_id', column2='user_id',
+                                string='Assignees', readonly=True)
     date_assign = fields.Datetime(string='Assignment Date', readonly=True)
     date_deadline = fields.Date(string='Deadline', readonly=True)
     partner_id = fields.Many2one('res.partner', string='Customer', readonly=True)
@@ -59,7 +60,6 @@ WITH all_moves_stage_task AS (
            COALESCE(LAG(mm.date) OVER (PARTITION BY mm.res_id ORDER BY mm.id), pt.create_date) as date_begin,
            mm.date as date_end,
            mtv.old_value_integer as stage_id,
-           usr_rel.user_id,
            pt.date_assign,
            pt.date_deadline,
            pt.partner_id
@@ -71,7 +71,6 @@ WITH all_moves_stage_task AS (
       JOIN ir_model_fields imf ON mtv.field = imf.id
                               AND imf.model = 'project.task'
                               AND imf.name = 'stage_id'
-      JOIN project_task_user_rel usr_rel ON pt.id=usr_rel.task_id
      WHERE pt.active
 
     --We compute the last reached stage
@@ -83,7 +82,6 @@ WITH all_moves_stage_task AS (
            COALESCE(md.date, pt.create_date) as date_begin,
            (CURRENT_DATE + interval '1 month')::date as date_end,
            pt.stage_id,
-           usr_rel.user_id,
            pt.date_assign,
            pt.date_deadline,
            pt.partner_id
@@ -99,7 +97,6 @@ WITH all_moves_stage_task AS (
                        AND mm.model = 'project.task'
                   ORDER BY mm.id DESC
                      FETCH FIRST ROW ONLY) md ON TRUE
-      JOIN project_task_user_rel usr_rel ON pt.id=usr_rel.task_id
      WHERE pt.active
 )
 SELECT (task_id*10^7 + 10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
@@ -108,7 +105,6 @@ SELECT (task_id*10^7 + 10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
        display_project_id,
        stage_id,
        d as date,
-       user_id,
        date_assign,
        date_deadline,
        partner_id,
@@ -125,7 +121,6 @@ SELECT (task_id*10^7 + 2*10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
        display_project_id,
        stage_id,
        date_trunc('week', d) as date,
-       user_id,
        date_assign,
        date_deadline,
        partner_id,
@@ -144,7 +139,6 @@ SELECT (task_id*10^7 + 3*10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
        display_project_id,
        stage_id,
        date_trunc('month', d) as date,
-       user_id,
        date_assign,
        date_deadline,
        partner_id,
@@ -163,7 +157,6 @@ SELECT (task_id*10^7 + 4*10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
        display_project_id,
        stage_id,
        date_trunc('quarter', d) as date,
-       user_id,
        date_assign,
        date_deadline,
        partner_id,
@@ -182,7 +175,6 @@ SELECT (task_id*10^7 + 5*10^6 + to_char(d, 'YYMMDD')::integer)::bigint as id,
        display_project_id,
        stage_id,
        date_trunc('year', d) as date,
-       user_id,
        date_assign,
        date_deadline,
        partner_id,
diff --git a/addons/project/report/project_task_burndown_chart_report_views.xml b/addons/project/report/project_task_burndown_chart_report_views.xml
index 74a519cc9b2..3cb97e0d8e8 100644
--- a/addons/project/report/project_task_burndown_chart_report_views.xml
+++ b/addons/project/report/project_task_burndown_chart_report_views.xml
@@ -7,12 +7,10 @@
         <field name="arch" type="xml">
             <search string="Burndown Chart">
                 <field name="project_id" />
-                <field name="user_id" />
                 <field name="date_assign"/>
                 <field name="date_deadline"/>
                 <field name="partner_id" filter_domain="[('partner_id', 'child_of', self)]"/>
                 <field name="stage_id" />
-                <filter string="Unassigned" name="unassigned" domain="[('user_id', '=', False)]"/>
                 <separator/>
                 <filter name="filter_date" date="date" string="Date" />
                 <filter name="filter_date_deadline" date="date_deadline"/>
@@ -23,7 +21,6 @@
                     <filter string="Date" name="date" context="{'group_by': 'date'}" />
                     <filter string="Stage" name="stage" context="{'group_by': 'stage_id'}" />
                     <filter string="Project" name="project" context="{'group_by': 'project_id'}" />
-                    <filter string="Assigned To" name="user" context="{'group_by': 'user_id'}" />
                 </group>
             </search>
         </field>
diff --git a/addons/project/wizard/project_task_type_delete.py b/addons/project/wizard/project_task_type_delete.py
index 0fbf271f067..573473464c9 100644
--- a/addons/project/wizard/project_task_type_delete.py
+++ b/addons/project/wizard/project_task_type_delete.py
@@ -58,7 +58,7 @@ class ProjectTaskTypeDelete(models.TransientModel):
             action = self.env["ir.actions.actions"]._for_xml_id("project.action_view_task")
             action['domain'] = [('project_id', '=', project_id)]
             action['context'] = str({
-                'pivot_row_groupby': ['user_id'],
+                'pivot_row_groupby': ['user_ids'],
                 'default_project_id': project_id,
             })
         elif self.env.context.get('stage_view'):