90s heartthrob shocks fans with grown-up photos of handsome look-alike son

Chris O'Donnell has a total twin in his house.

On Saturday, October 24, the 9-1-1: Nashville star rang in his look-alike son Chip's 26th trip around the sun with a round of photos of the two.

In addition to the birthday boy, the NCIS: Los Angeles alum, who has been married to his wife Carolien Fentress since 1997, is also a dad to Lily, 27, Charles, 23, Finley, 20, and Maeve, 18.

In honor of Chip's special day, Chris, 56, took to Instagram and first shared a selfie of the two — Chip looking so much like his dad it looked like a blast from the past.

The doting dad next shared throwback photos of Chip growing up, as well as some more recent photos that further highlighted how much they look alike.

"26 years of dominating you on the golf course, happy birthday @chipodonnell love you guy!" he wrote in his caption, and friends and fans alike were quick to take to the comments section under the post and gush over it, marveling over how fast he has grown up.

"26!!!?!?????" fellow NCIS: Los Angeles alum Eric Olsen exclaimed, as others followed suit with: "How do you have a 26 year old??" and: "Happy Birthday to your twin! Also, love the glasses!" as well as: "Damn. It's like he multiplied. Beautiful."

Chris and his family have been partly based out of Nashville since last year, when he started working on the 9-1-1 spin-off based in the city, which also stars Jessica Capshaw, Leann Rimes, Kimberly Williams-Paisley, Michael Provost, Tim Matheson, Gregory Alan Williams, and MacKenzie Porter.

Earlier in October, he also launched a new production company, Lone Cypress Entertainment, which will debut with a film titled Kilkenny, starring P.J. Byrne and will be directed by Jay Duplass.

"When we wrapped NCIS: Los Angeles after 14 seasons, it coincided with our business being in the midst of great disruption," he shared with Deadline, seemingly referring to the months-long SAG-AFTRA strike that began one month after NCIS: Los Angeles' finale. "As I thought about the next chapter of my career, I kept coming back to the desire to work on unique projects with talented people."

He continued: "When I was starting out, I was fortunate to work with incredible actors and directors who took me under their wings and taught me the ins and outs of the business from performing to promoting."

"Part of my inspiration for creating Lone Cypress is to give back to the next generation of talent. At the same time, there are so many talented actors and filmmakers I came up with and who blazed the trail before me that I would love to collaborate with."