The apparent vandalism was discovered around 1:30 a.m. Friday on the statue, the pedestal and the floor, U.S. Park Police said. No words, letters or symbols were visible in the paint.
The marble Lincoln statute had green paint on its shin, coattail, chair and base, as well as paint on the floor of the memorial building.
Capt. Steven Booker said the paint spill "appears intentional based off of the splatter." Police were reviewing security camera footage to try to identify possible suspects, he said.
No suspects had been identified by Friday afternoon. Police officials said they would not release the security footage because the investigation is ongoing.
The memorial chamber was closed all day to allow a National Park Service crew to finish cleaning up the paint. Workers spent hours using pressurized hoses and a chemical paint remover to try to wash away the paint.
National Park Service spokeswoman Carol Johnson said the memorial will be returned to the condition it was in before the vandalism. She said the work was going well by late Friday.
"It is not permanent damage," she said. "Our historic preservation crew knows exactly what they need to do."
The memorial, one of the most popular sites on the National Mall, was dedicated in 1922 to President Abraham Lincoln. The building was designed by Henry Bacon, and Daniel French sculpted the statue of Lincoln. It sits at the opposite end of the National Mall from the Capitol, facing the Washington Monument.
The memorial has served as a symbol of equality and reunification after the Civil War. It was the site of the Rev. Martin Luther King Jr.'s March on Washington 50 years ago, along with other historic moments.
The Lincoln Memorial is generally open around the clock to visitors. Park rangers leave their posts about 10 p.m. and return about 9 a.m. daily. U.S. Park Police, however, maintain 24-hour patrols at the memorial, said Lt. Pamela Smith.