A first-half Romelu Lukaku strike and a second-half own goal from the Swans' Dutchman earned the home side only their third win of 2013 to keep their Europa League hopes alive
Early pressure from Swansea paid off after 33 minutes when Luke Moore headed his side into the lead, but the hosts immediately sprung into life and Lukaku equalised just before half-time after a swift Baggies counterattack.
Lukaku missed the opportunity to put his side ahead from the penalty spot after 56 minutes, but De Guzman was on hand to accidentally put the ball in to the back of his own net five minutes later to wrap up all three points for West Brom.
Baggies manager Steve Clarke was able to welcome back star striker Lukaku, who was ineligible to play against his parent club Chelsea last weekend, as well as Chris Brunt, who did not feature at Stamford Bridge due to an ankle injury. Shane Long, also suffering with an ankle injury, was absent.
Swansea boss Michael Laudrup had just one fresh injury to worry about, with Leon Britton missing out due to a hip injury and being replaced by Moore. The Danish manager made one other change to the side to the one that won 1-0 against Newcastle last week, with Sung-Yeung Ki coming in for Nathan Dyer.
The Swans arguably started the stronger side, and both Michu and De Guzman were able to carve out good chances on goal early on, but the West Brom defence held firm to deny their visitors going ahead.
Swansea's constant pressure finally paid off after 33 minutes when Moore headed his side into the lead, the former West Brom player escaping Jonas Olsson in the West Brom defence to divert De Guzman's corner into the back of the net for his second goal in as many games.
Despite being second-best for the vast majority of the first half, the Baggies managed to find an equaliser seven minutes later through Lukaku, after a swift counterattacking movement down the right wing allowed Graham Dorrans to cross in for the Belgian teenager to poke past Swansea goalkeeper Michel Vorm.
Ten minutes into the second half, the youngster had the opportunity to put his side ahead from the penalty spot after Wayne Routledge was fouled on the edge of the area by James Morrison, but a poor spot kick from the Belgian was comfortably saved by Vorm.
West Brom seemingly were not fazed by the miss, however, and it only took them five minutes to go ahead when Angel Rangel cleared a Gareth McAuley headed effort off the line, only for his clearance to rebound off the back of De Guzman and into the corner of the net.
Swansea thought that they had rescued themselves a point in the final minutes when Roland Lamah hit the back of the net with only three minutes left to play, but the linesman flagged for offside, much to the relief of West Brom, despite replays showing that the ball did not touch another Swans player in the build-up to the goal.