The NCIS star was spotted driving into Los Angeles' Cedars-Sinai Medical Center, where the pop star is receiving treatment.
The outlet reports that Valderrama arrived at approximately 3 p.m. on Wednesday and a security guard ushered him into the staff parking lot on site.
Shortly before his visit, Valderrama was spotted grabbing lunch in Encino, California, located about 17 miles from the hospital. The actor refrained from showing emotion as he waited in a parking lot wearing a chambray shirt, dark jeans and boots.
An eyewitness told the outlet, "He was serious, intense and seemed distant, but focused and very calm at the same time."
Valderrama is the first celebrity guest to visit the pop star in the hospital. Her mother, Dianna De La Garza, and her sisters have been by her side since Tuesday afternoon.
The couple dated for six years and called it quits in June 2016, the two have remained close friends despite their split.
"Even after they broke up, they continued to be very good friends," a source told Entertainment Tonight. "They have mutual admiration for each other and he has and will always believe in her. He's watched her fall and has seen her rise."
"[Wilmer] was aware she was going through a bumpy patch because she was forthright with him when she relapsed," the source added. "But he did not anticipate this."
Another source told the outlet that the actor is concerned about her health following the incident.
"Wilmer is completely devastated and he still loves Demi and it was just a matter of scheduling and time that they needed to make a romantic relationship work," a source said. "She's been honest with him about her relapse, as she has with her family, friends, fans and the entire public, especially with her new song, but he didn't think it was this severe and she would be struggling so much and have a situation that had these repercussions."
Lovato was rushed to the hospital Tuesday before noon after she was found unconscious, and was reportedly revived with Narcan, a medication to counteract the effects of opioids.
"Demi has the love and support of her family," a source told the outlet. "Right now their plan is to keep her away from those people in her life that don't support a clean and sober future."
Hoping for a quick road to recovery for Demi!