Party members and supporters have been choosing between Anas Sarwar and Richard Leonard.
The two MSPs announced their candidacies after Kezia Dugdale quit the top job in August.
Mr Sarwar has the backing of several of the party’s high profile parliamentarians, including MP Ian Murray and current interim leader Jackie Baillie, while the more left-wing Mr Leonard has the support of the majority of trade unions.
Glasgow MSP and party health spokesman Mr Sarwar came under close scrutiny early on in the leadership contest for the working practices at his family’s firm.
Meanwhile, supporters of both candidates raised concerns over the sign-up process for those eligible to vote, with accusations of unfair tactics on both sides.
The leadership contest concludes at the tumultuous time for the party, with Alex Rowley being suspended as a member of the Scottish Labour Parliamentary group on Wednesday.
Mr Rowley had already stepped down as interim and deputy leader after allegations about his conduct emerged.
Scottish Labour bosses will announce the winner of the leadership contest in Glasgow on Saturday.