Waseem Baloch, 25, was arrested in Dera Ghazi Khan in central Pakistan on Saturday night.
The Dawn newspaper said he had confessed to the murder, saying he drugged and strangled her "for dishonouring the Baloch name".
Qandeel Baloch, 26, became a household name for posting sometimes raunchy photographs, comments and videos.
She recently caused controversy by posting pictures of herself alongside a Muslim cleric.
Dawn quoted police as saying her selfies with Mufti Qavi were one of the issues cited by her brother.
The paper quoted Waseem Baloch as saying: "She wasn't aware I was killing her. I gave her a tablet and then strangled her."
Police said the brother had escaped to Dera Ghazi Khan after the killing in the Karimabad area of Multan early on Saturday morning.
They said he fled with two friends who were still being sought.
Ms Baloch was buried on Sunday morning in her ancestral village near Dera Ghazi Khan in Punjab province.  Images from the scene showed scores of people attending the services.
Mufti Qavi, who said he had forgiven Ms Baloch for her actions, had offered to lead the funeral prayers, reports said.
Ms Baloch had built up a large social media fan-base, with 43,000 Twitter followers and more than 700,000 on Facebook.
She was well aware of the opposition she faced but continued to post defiant tweets. One on Thursday read: "I will not give up. I will reach for my goal & absolutely nothing will stop me."
Ms Baloch, dubbed by some Pakistan's Kim Kardashian, rose to fame in 2014 when a video of her pouting at the camera and asking "How em looking?" went viral.
In a recent interview she was bitterly critical of Pakistan's patriarchal society and described herself as a leading exponent of girl power.
While many younger people saw her as a cultural icon and hailed her liberal views, she was also subjected to frequent misogynist abuse online.
Ms Baloch had gone to Punjab from Karachi because of the threat to her security, police say.
Her request for better security was ignored by the government, Dawn reported, despite pleas made three weeks ago to the interior minister and other senior officials.
Hundreds of women are murdered every year in Pakistan in so-called honour killing cases.
Qandeel Baloch used social media to find fame and the reactions there showed the feelings she inspired, from admiration to disgust.
Some called her death "good news" and even praised her suspected killer. Others said it was wrong to condone her murder, even if she was flawed. Some showed outright support.
Qandeel Baloch has been dubbed Pakistan's Kim Kardashian. There are comparisons: the provocative selfies, the pursuit of celebrity, the controversial rise to notoriety.
But in Pakistan, women, especially poor ones, still lack basic rights, from schooling to choosing a husband and violence against them is rife. The country struggles with sexuality and especially with "immodest" women.
The fact that many of her videos went viral suggests a titillating fascination with confident female sexuality - along with fear of its power and of her assertion of independence. However she lived her life, tweeted one, it was her life.
The brother of Pakistani social media celebrity Qandeel Baloch has been arrested for her murder.