The on-loan 24-year-old missed last weekend's trip to parent club Liverpool and made an immediate impact on his return to Sam Allardyce's starting line-up. Having proved a nuisance all afternoon, the £35million striker fired low past Artur Boruc to deny Southampton a fourth win on the bounce.
Saints came closest to breaking the deadlock in a nondescript first half, with Jussi Jaaskelainen somehow tipping a close-range Rickie Lambert effort onto the post.
Carroll fired just wide 10 minutes into the second half and the hosts soon capitalised, with club record signing Gaston Ramirez netting.
The lead was to last just seven minutes, though, as Carroll found the net after firing home a short free-kick from Gary O'Neil to earn the east Londoners a point at their fellow promoted side.
Adam Lallana was quick to make an impact in a week in which he penned a new five-year deal, being denied inside two minutes by a late James Tomkins challenge.
Tomkins then reacted well to thwart Lallana from close range when play returned to the other end, with Jaaskelainen then superbly denying Lambert. Ramirez floated in a free-kick to the back post and the Saints forward prodded goalwards, only for the Hammers goalkeeper to superbly tip onto the post.
It took until the 55th minute for the first chance of note, with Carroll flashing a strike inches wide after Saints failed to clear their lines.
The hosts took advantage of that miss by taking the lead just three minutes later. Boruc's long ball up field was only cleared by Tomkins as far as Ramirez, who went through to net his fifth of the season. After the ball squirmed under a joint challenge from Collins and Tomkins, he coolly fired home past Jaaskelainen.
However, the 2,740 visiting fans did not have to wait long for something to cheer about as West Ham won a free-kick on the edge of the box and O'Neil played it short for Carroll to fire low past Boruc via a slight deflection off Steven Davis.