Mike Dyer, constituency chairman of Anniesland, in Glasgow, revealed his backing for a "Yes" vote in the referendum at the STUC conference in Dundee.
He cited the "relentless negativity" of the No campaign as one of the key motivating factors in his decision to back the nationalist drive.
SNP deputy leader Nicola Sturgeon made a special appeal to Labour voters to back independence in her conference speech on Friday, amid a concerted push to make pro-independence group Yes Scotland more appealing to supporters of the unionist party.
Mr Dyer, a member of Unite, said he believes independence will lead to a "fairer Scotland".
He said: "With a Yes vote industrial relations will no longer be shackled by the same anti-strike regulations or restrictions on employees' rights that stem from Westminster.
"In an independent Scotland, workers can expect greater legal protection where we'll all have collective bargaining rights and the right not to be victimised.
"Trade unionists want a Scotland where workers are looked after and a Yes vote is the best way to achieve that.
"The No campaign has so far been a tide of relentless negativity."