The 26-year-old has agreed a two-year deal at Fratton Park after his contract with the relegated Bees expired.
"It's taken some time to get the deal over the line but now I'm just excited about getting out on the pitch and playing in front of the crowd," Holmes told Pompey's website.
"There were other clubs interested in me, but signing for a side like Pompey is a dream come true.
"The history of this club is known across the world, so it wasn't really a hard decision.
"Hopefully I can get the crowd onto their feet. I work hard, dribble the ball, get past defenders and try to excite the crowd."