Mark Zuckerberg had the best view of anyone at Donald Trump's inauguration ... because he was sitting next to Lauren Sanchez -- and staring down at her chest!!!

The Meta owner got caught on camera eyeballing Sanchez's assets while attending Monday's swearing-in of DT as the 47th President in the Capitol Rotunda, along with the world's biggest power-brokers.

Zuck was also seated near Sanchez's fiancé, Amazon owner Jeff Bezos, and Trump's son-in-law Jared Kushner.

Check out the video circulating on X ... Jared is chatting with Bezos as Zuck nervously rocks back and forth, peering down -- TWICE -- at Sanchez's boobs in a white lacy corset under a matching white coat.

Sanchez seems completely oblivious to Zuck's wandering eyes, smiling at one point as she takes in Jared's conversation with her man.

Even though Zuck is all about MMA fighting these days, he better watch his step with Bezos, who probably won't like his tech rival sizing up his babe.

Keep looking forward next time Zuck!