The liberal Freedom Egypt party, headed by Amr Hamzawy, has said it will support Hamdeen Sabahi in the presidential election, Aswat Masriya reported on Tuesday.
In a statement, the party said the decision came after an internal vote and despite the absence of a candidate whose views fully match its own.
Leftist politician Sabahi is the only challenger to former military chief Abdel-Fattah El-Sisi, who is expected to win by a landslide in the vote on 26/27 May.
The party said it would "support the candidate with views closest to those of the party and who will achieve the goals of the Egyptian revolution and preserve its principals."
The Constitution, Socialist Popular Alliance, Popular Current, and Justice parties have also come out in support of Sabahi
Others including the Wafd, Free Egyptians, Tagamuu and Conference parties have backed El-Sisi.
Hamzawy, a political science professor and former MP, has criticised the government installed after the July ouster of Islamist president Mohamed Morsi for it breaches of human rights.
"[The party] has taken the approach of positive interaction with the (political) roadmap with reservations on any step or practice that has deviated from the democratic path we want for Egypt," the statement said.
Freedom Egypt secretary-general Shaheer George told Ahram Online that the party had reservations over the presidential election law, which places the election committee above judicial review.
"The general atmosphere of arrests and the protest law gives the impression that the upcoming election might not be fair and transparent," George said. "But we found it necessary to participate and interact."