Bath are currently bottom of the Premiership with only six games to go, and with automatic relegation looming this time round, their decision to snap up the versatile 12-cap three-quarter demonstrates the importance of their survival fight.
Seveali'i, who made his Wellington Lions debut back in 1999 when they faced Manawatu, is a current international with Samoa having made his debut against Japan in 2000, and can play in the centres as well as out wide.
He joins a Bath backs division with quality aplenty in the centre department, Ireland's Kevin Maggs and Bath's Mike Tindall forming part of a squad which also sees Olly Barkley and Mike Catt present.
On the wing the West Country side have the likes of England's Iain Balshaw and Scotland cap Simon Danielli, although co-coach Michael Foley has just released England Sevens star Rob Thirlby to Division One side Rotherham.
Bath Director of Rugby Jack Rowell said of Seveali'i's signing: "Elvis will immediately provide experienced cover in the centre/wing positions and should indeed go on to challenge for first team selection. Depending on his progress, we may well extend his stay with Bath Rugby into next season."
Co-coach Brian Smith added: "Elvis will be joining us on a trial basis. He has presented his credentials well, playing in the NPC Championship with Wellington, and will add strength to our squad with Iain Balshaw sidelined through injury and the current international demands.
"We are excited about him being here and look forward to seeing some good things from him."
Seveali'i himself commented: "Bath Rugby is a special club full of quality players and I am really looking forward to training and playing with them as well as the challenge of what is an extremely competitive Zurich Premiership."
Bath are next in action this Saturday when they host Saracens in a vital Zurich Premiership clash.
By Mark Smith