The 27-year-old goalkeeper is to link up with Orient on a two-year contract after seeing his deal at Derby expire.
He has spent the last four years in the Midlands, making 37 first team appearances.
Legzdins saw no Championship action in the 2013/14 campaign, though, behind Lee Grant and has been reluctantly forced to accept that he needs to take his career in a different direction.
He said in the Derby Telegraph: “I leave Derby with a heavy heart. It was a great set of players and I was friends with all of them.
“I can see how that togetherness can help you achieve great things and so I wanted a club with a close set of lads with great camaraderie.
“Last year was disappointing for me for various reasons and I played fewer games than I’d hoped, but the year before I played 33 games in the Championship.
“That’s my ambition, to play games. I don’t want to be number two goalkeeper.”
Legzdins was a loan target for Orient last season and both parties are delighted to have seen a deal belatedly pushed through.
He added: “I’m delighted that it’s all been sorted.
“I was aware that Orient were interested in me last year for a loan move but, unfortunately, it didn’t happen.
“But once I found out they were interested in me again, I spoke to the manager and the goalkeeper coach and it became very clear the ambition of the club is to build on last year, so I’m thrilled to be a part of it.”