The team said the player, who was not identified, entered self-quarantine immediately.
"This evening, we learned a Rams player tested positive for COVID-19. The player immediately entered self-quarantine and out of an abundance of caution, we are entering intensive protocol," a statement said.
All football activities will take place remotely on Wednesday, except for select players with assigned rehab, according to the team.
Players and coaches will conduct their normal meeting schedule from home.
"This decision was made in consultation with the NFL and medical officials as the health and safety of the team is our highest priority," the statement said.
The team had previously been struck with two COVID-19 cases. Earlier this year, center Brian Allen and tackle Andrew Whitworth tested positive for the virus.
RELATED: Rams tackle Andrew Whitworth and wife open up about family's experience with COVID-19
Rams tackle Andrew Whitworth and wife open up about family's experience with COVID-19
The Rams (6-3) are scheduled to play the Tampa Bay Buccaneers (7-3) on Monday. According to ESPN, Tuesday was a regularly scheduled day off for the team and they were set to return to the practice facility Wednesday for a walk-through, which is now canceled.
The Rams have not had a positive case since rookie outside linebacker Terrell Lewis, when he reported to training camp.
ESPN contributed to this report.