Walton learned the woman was pregnant two days before his arrest, the Broward County Sheriff's Office said.
Walton is charged with aggravated battery on a pregnant woman, according to court records. That is a second-degree felony that carries a maximum of 15 years in prison, the Sun-Sentinel reported.
Walton will remain in jail until his preliminary hearing Wednesday morning in a Broward County courtroom, the newspaper reported.
"We were made aware of a police matter earlier this morning regarding Mark Walton," Dolphins general manager Chris Grier said in a statement. "We hold our players to a high standard and take these matters very seriously. We will have no further comment at this time."
"I think it's an unfortunate situation," Walton's attorney Michael A. Gottlieb, of Fort Lauderdale, told the Sun-Sentinel on Tuesday. "It's obviously in the very early stages, and we're looking for a favorable outcome."
The Dolphins signed Walton in May. In seven games this season, he has rushed for 201 yards
Despite the off-field issues earlier this year, the Dolphins signed Walton shortly after a tryout in May and saw him turn into a capable starting NFL running back, who accounted for 53 carries and 201 yards rushing in seven games this season.
Walton was sentenced in August to six months of probation after he pleaded no contest for incidents that occurred in January, February and March, WSVN reported.
Walton was suspended by the team Nov. 4, the Sun-Sentinel reported. He was eligible to return to action Dec. 8.
Walton, a Miami native, played for the University of Miami in college. He was drafted by the Cincinnati Bengals in the fourth round of the 2018 NFL draft. He was released by the Bengals earlier this year.