Rodriguez was arrested and being held at the ballpark early Thursday, police said. His father-in-law was in a hospital with a scrape on his face and a bump on his head.
Rodriguez was expected to be arraigned Thursday morning at Queens Criminal Court. Under the domestic violence laws in New York state, Rodriguez was charged regardless of whether the father-in-law wished to bring charges.
"There was an incident at the ballpark between Francisco Rodriguez and his family. He was questioned by police and all other questions pertaining to this matter can be addressed to police," Mets spokesman Jay Horwitz said.
Several security guards, along with some women and children, were seen around the Mets' family lounge near the clubhouse after New York's 6-2 loss to Colorado. Rodriguez's father-in-law had swelling above his right eyebrow.
Rodriguez became highly agitated with reporters who approached him at his locker after the altercation.
Rodriguez screamed, "Mind your own [expletive] business!" to a reporter who tried to ask him a question. The reliever then stormed into a restricted area of the clubhouse. Soon after, a Citi Field security member and uniformed police officer went to that same area of the clubhouse.
Rodriguez has had previous confrontations with a team official, a coach and an opponent in his 1½ seasons with the Mets, though none of them reached this level.
The excitable 28-year-old pitcher signed a three-year, $37 million contract with the Mets after posting a record 62 saves with the Angels in 2008. He is 4-2 with 25 saves and a 2.24 ERA this season.
In May, Rodriguez and bullpen coach Randy Niemann got into a heated exchange while the pitcher was preparing to enter a game. Mets manager Jerry Manuel later said the matter was resolved, without divulging what caused the dispute.
In July 2009, Rodriguez and former Mets official Tony Bernazard argued aboard the team bus. Bernazard was later fired by the Mets amid several allegations.
Information from ESPN's Jeremy Schaap, ESPNNewYork.com's Ian Begley and The Associated Press was used in this report.