This is The Atom, aka Ray Palmer.
This is The Atom, aka Ryan Choi. Choi first appeared after Ray Palmer disappeared following the events of Identity Crisis (2004).
The CW introduced Ray Palmer to the Arrowverse in the season 3 premiere of Arrow (October 2014). They originally wanted to use Ted Kord/Blue Beetle, but DC said he was tied up elsewhere. So they basically did a find-and-replace on all instances of "Ted Kord" in the script and called it a day. Instead of being a science professor at Ivy University, Ray Palmer is a technology CEO who builds himself an exosuit.
And if you think I'm being too harsh, check this out: Ray Palmer uses the ATOM suit in six episodes of Arrow season three and one episode of The Flash season one. We don't find out it can shrink until Arrow season four.
Anyway, last year DC ushered in Rebirth, which was supposed to accomplish several different things, one of which was bringing back fan-favorite characters that the New 52 had shuffled off. One of these was Ryan Choi, who appeared in DC Rebirth #1 (May 2015).
As you can see, Ray Palmer is still wearing his classic costume. But when Ryan Choi debuted (again) at The Atom in Justice League of America: The Atom #1 (January 2017), it was in a costume very reminiscent of the tv version.