The five missing liberal activists, some of whom have posted blogs criticising the political influence of the military and speaking up for the rights of religious minorities, had each gone missing separately since January 4.
Police sources told Geo News channel that Haider, who disappeared on January 6, was found late on Friday night.
"Police sources have confirmed that he has been returned and also that his physical condition is okay," Geo News reported on Saturday, but giving no further details on how Haider was found.
"Police say he was returned to Islamabad last night."
Haider's family could not be immediately reached for confirmation. There was no word on the whereabouts of the four other missing activists.
It is not known how the five activists went missing, but some rights groups and newspapers have asked whether state or military agencies were in any way involved.
The Interior Ministry has repeatedly said it is doing all it can to recover the missing men.
Shortly after the activists' disappearances, blasphemy allegations against them appeared on social media and in a complaint to police.
In Pakistan, conviction under the blasphemy laws can carry a mandatory death sentence.
Haider has written columns for a popular English-language newspaper and taught at the Fatima Jinnah Women's University in the city of Rawalpindi, some 15km from capital Islamabad.
Last year, Haider wrote a poem about human rights abuses in Pakistan's restive Baluchistan province, including a line about his friends' friends disappearing. He queried whether his friends, or even he himself, will be next to suffer such a fate.
Two of the missing activists, Waqas Goraya and Aasim Saeed, live in the Netherlands and Singapore. Their relatives said they were taken on January 4 while visiting Pakistan. The fourth activist, Ahmed Raza Naseer, suffers from polio.
A fifth Pakistani social activist, Samar Abbas, went missing from the capital Islamabad on January 11.
(Reporting by Saad Sayeed and Mehreen Zahra-Malik; Writing by Mehreen Zahra-Malik; Editing by Tom Hogue)