Johnson will be a welcome arrival at Upton Park with centre-backs Winston Reid, James Collins and James Tomkins all currently unavailable due to their respective injuries.
But the 30-year-old, who has spent the first half of the season at Sky Bet Championship club Sheffield Wednesday, eases the crisis somewhat and the Hammers confirmed he will be available for Wednesday's Capital One Cup semi-final first leg clash at Manchester City.
Johnson will bring a wealth of experience to the London club, having made over 100 Premier League appearances with Birmingham, with whom he won the League Cup in 2011, and Wolves.
The centre-back moved to Blues' midlands rivals Wolves in July 2011 following Birmingham's relegation and was immediately handed the captaincy by then-manager Mick McCarthy.
But following McCarthy's sacking, Johnson found himself marginalised and was disciplined by the club for allegedly training under the influence of alcohol, while he also had an on-field dispute with team-mate Wayne Hennessey as Wolves were relegated from the Premier League.
Johnson had the unwanted tag of being relegated for the third season in a row when Wolves dropped down into League One last season, and he found himself out of favour with new boss Kenny Jackett.
Johnson joined Sheffield Wednesday on a three-month loan in September although the Owls opted not to extend the deal, which has allowed the Hammers to come in.