Turner was pulled during the 7th inning of Game 6 on Tuesday and told to isolate -- but MLB says he "emphatically refused to comply" and returned to the field to celebrate after the Dodgers defeated the Tampa Bay Rays.

Turner was seen hugging and interacting with several players, staffers and family members ... at one point he sat down next to Dodgers Manager Dave Roberts (a cancer survivor) without a mask on his face.

MLB is now going hard at Turner for his conduct -- and announcing a full investigation.

"Immediately upon receiving notice from the laboratory of a positive test, protocols were triggered, leading to the removal of Justin Turner from last night’s game," MLB says.

"Turner was placed into isolation for the safety of those around him. However, following the Dodgers’ victory, it is clear that Turner chose to disregard the agreed-upon joint protocols and the instructions he was given regarding the safety and protection of others."

"While a desire to celebrate is understandable, Turner’s decision to leave isolation and enter the field was wrong and put everyone he came in contact with at risk.  When MLB Security raised the matter of being on the field with Turner, he emphatically refused to comply."

MLB says the Commissioner’s Office is beginning a "full investigation into this matter and will consult with the Players Association within the parameters of the joint 2020 Operations Manual."

MLB says players from the Rays and Dodgers were tested directly after the game on Tuesday and again Wednesday.

Travel plans for the teams to return home will be contingent on the results of those tests.

"Travel back to their home cities will be determined after being approved by the appropriate authorities.”