Brighton moved into a play-off position after midfielder David Lopez's second goal of the season ended Birmingham's 10-match unbeaten run.
Lopez struck after the interval, the 31-year-old Spaniard finding the bottom corner of the net after Andrew Crofts had seen his shot parried.
Birmingham had their chances, Chris Burke firing over before Kazenga LuaLua and Burke were both denied.
But Brighton deserved their win, with Crofts and Lopez both going close.
The Seagulls have lost only one of the last seven outings and are now five points behind fourth-placed Derby.
Birmingham, who failed to score for the third time in five games, are 17th in the Championship table, eight points above the relegation zone.
Blues boss Lee Clark can have few complaints, with the hosts managing 24 shots compared to seven by the visitors.
From Basque Country to Brighton
David Lopez began his career at Osasuna and moved to Athletic Bilbao for €6m in 2007 but left after struggling for first-team opportunities. He has 11 goals in 59 appearances for Brighton.
Lopez struck just before the hour mark, the former Athletic Bilbao man pouncing after Crofts forced Darren Randolph to parry his shot.
Striker Jonathan Obika, signed on a 93-day loan from Tottenham, came on for his Seagulls debut with 25 minutes remaining
And the 23-year-old helped set up a chance for Lopez to double the lead but this time he could not find the net.
Birmingham rallied, former Manchester United goalkeeper Tomasz Kuszczak denying Burke an equaliser with a late save.
Brighton manager Oscar Garcia:
"Yes, we are sixth, but I am pleased because we won the match and once again the performance of the team was very good.
"That is all, I don't want to comment further about our position in the league.
"I asked the players to be patient. We had to show Birmingham some credit as they were unbeaten in their last ten matches."
Birmingham manager Lee Clark:
"We were a bit frustrated with our work in the final third.
"The quality of our final pass was not up to the required standard.
"We weren't clinical enough but the players have shown me terrific character."
Brighton & Hove Albion 29 Kuszczak 02 Bruno 17 Ward 04 Andrews Booked 03 Greer 20 Upson 08 Crofts (Orlandi - 82' Booked ) 38 Ince Booked 19 Ulloa (Obika - 63' ) 21 D. López (El-Abd - 91' ) 25 Lua Lua Substitutes 01 Brezovan 06 El-Abd 10 Agustien 11 Orlandi 14 Calderón 22 Obika 27 March Birmingham City 01 Randolph 31 Caddis 18 Hancox 17 Reilly Booked (Ferguson - 82' ) 08 Mullins Booked 04 Robinson 07 Burke 20 Lee Booked 19 Zigic 15 Elliott (Brown - 63' ) 12 Novak Substitutes 13 Doyle 14 Packwood 22 Shinnie 27 Ferguson 29 Brown 32 Bell 33 Gray
Ref: Roger East