The Barcelona star was set to complete a permanent switch to East London this summer after impressing while on loan last season.
But an injury which required surgery prevented the Hammers from making the move permanent, opting instead to sign the former Arsenal midfielder on loan for a second season.
Should Song return to fitness and form – he is expected back in and around the first team picture next month – both the club and player have already stated a desire to finally make the deal permanent.
'I want to be part of that story'
"It's good to be here, I want to just come back (now) because the future of the club is very big," Song told Sky Sports.
"That's why I said last year, in the next five years I think West Ham will be a very big club in London and I want to be part of that story."
Hammers could be even more on Song with Alex
Song was head and shoulders West Ham's best player as they made their best ever start to a Premier League season last year.
Sitting in third place at the turn of the year under then manager Sam Allardyce, the Hammers were proving difficult to deal with as Song pulled the strings in midfield.
The team's form dipped alarmingly, though, as did the former Cameroon captain's performances and they finished a disappointing 12th.
Slaven Bilic made signing Song one of his priorities after taking over from Allardyce and eventually got his man on transfer deadline day.
Payet and Song a match made in heaven?
The Hammers have made an even more impressive start this season, their new stylish football propelling them up to third in the Premier League table with a chance to go top if they beat basement boys Sunderland at the weekend.
When Song returns he will only strengthen the high-flying Hammers even more and Bilic's side threaten to be a real force to be reckoned with.
The thought of the African star lining up alongside French wizard Dimitri Payet has Hammers fans dreaming big just like Song.