Kickoff is set for 1:15 p.m. ET, and the game will be broadcast nationally by NBC.
The decision came after the COVID-19 outbreak among the Ravens worsened, and it prompted an angry response from Steelers players.
Baltimore added three more players -- offensive linemen Patrick Mekari and Matt Skura and defensive end Calais Campbell -- to the reserve/COVID-19 list Wednesday. The Ravens now have seven players who tested positive or have been identified as close contacts, a group that has totaled 2,127 snaps this season.
The Ravens issued a statement later Wednesday, saying an unnamed staff member has been disciplined "for conduct surrounding the recent COVID-19 cases that have affected players and staff at the Ravens." The team did not provide additional details.
The NFL decided to postpone the game because it wants to use the rest of the week to conduct more testing and contact tracing, a source told ESPN's Dan Graziano. The announcement came about 31 hours before the scheduled kickoff.
"This decision was made out of an abundance of caution to ensure the health and safety of players, coaches and game day personnel and in consultation with medical experts," the NFL said in a statement.
The outbreak has been widespread in Baltimore, where members of the Ravens' coaching staff and support staff have also tested positive.
"We appreciate the NFL for its diligence in working closely with us to ensure the well-being of players, coaches and staff from both the Baltimore Ravens and Pittsburgh Steelers organizations," the Ravens said in an earlier statement.
This is the second time the Steelers' schedule has been affected by COVID-19. Their Week 4 game against the Tennessee Titans was moved to Week 7 because of an outbreak in the Titans organization. The first Ravens matchup was then moved from Week 7 to the Steelers' originally scheduled bye on Week 8, and the Week 4 postponement became Pittsburgh's de facto bye week.
This time, the Steelers lose their mini-bye week with the Thanksgiving game being moved to Sunday.
The Steelers will not host fans for Sunday's game because new orders and advisories from the state of Pennsylvania regarding limits on outdoor gatherings go into effect Friday. Only friends and family of players and the organization will be allowed in the seating bowl for a full capacity at Heinz Field of 2,500, including coaches and players.
Steelers wide receiver JuJu Smith-Schuster tweeted within minutes of Wednesday's announcement of the rescheduled game, expressing his disappointment.
First the NFL takes away our bye week because another team can't get their Covid situation together, now they take away our Thanksgiving primetime game for the same reason. Smh.— JuJu Smith-Schuster (@TeamJuJu) November 25, 2020
Sources told ESPN that the Steelers will have a players day off Thursday after practicing Wednesday. Coach Mike Tomlin had planned to give his players off until next Wednesday after the Thanksgiving game, but those plans have changed.
"We feel like we have a really good handle on exactly when the transmission occurred and how it occurred," Dr. Allen Sills, the NFL's chief medical officer, told NFL Network on Wednesday. "I think that we feel like we're a couple of days away from being out of that window of vulnerability for that transmission event."
This marks the 13th NFL game that has been postponed or moved this season because of coronavirus-related issues, but it is the first postponement since Week 5. Three weeks ago, the San Francisco 49ers played against the Green Bay Packers on Thursday night even though they had placed four players on the reserve/COVID-19 list.
Earlier Wednesday, some Ravens players expressed concern about playing.
This is the first time since 2005 that the NFL will have only two games played on Thanksgiving.
The Ravens' outbreak began hours after Sunday's 30-24 overtime loss to the Titans, when the team learned running backs J.K. Dobbins and Mark Ingram had tested positive. Nose tackle Brandon Williams was later identified as a high-risk close contact.
On Monday, Baltimore closed its facility in the morning before reopening in the afternoon, when the team held a walk-through. A day later, the Ravens shut down the facility again after more positive tests and conducted all team activities virtually. Outside linebacker Pernell McPhee became the fourth Baltimore player added to the reserve/COVID-19 list in two days.
The postponement of Baltimore's Thanksgiving game to Sunday means a quick turnaround. The Ravens are set to host the Dallas Cowboys on Thursday, Dec. 3. The Cowboys will have a full week to recover after playing on Thanksgiving, but Baltimore will play on three days' rest.
The Ravens could enter that game short-handed. Any players who test positive must quarantine for at least 10 days.
The Steelers, who are 10-0 for the first time in franchise history, can clinch a playoff berth with a victory over Baltimore and losses by the Miami Dolphins and Las Vegas Raiders. The Ravens (6-4) are looking to avoid their first three-game losing streak with Lamar Jackson as their starting quarterback.