"They have broken up, but are still friends," Valderrama's publicist, Heidi Slan, told The Associated Press Friday. She declined to provide details.
Lohan and Valderrama acknowledged being a couple in July, after Lohan turned 18.
The couple's breakup was reported Friday by People and Us Weekly magazines.
Lohan's publicist, Leslie Sloane Zelnik, didn't immediately return a call seeking a comment from the AP. Us Weekly quoted Zelnik as saying, "I can confirm that they have amicably split, but the two remain close friends and supportive of each other's busy careers."
Valderrama stars on the Fox sitcom "That '70s Show."
Lohan, who also starred in "Freaky Friday," was hospitalized in Los Angeles in late October for a high fever and headaches. She became ill while filming the movie "Herbie: Fully Loaded."