He vowed to form a federal police unit to deal with violence against women.
The girl, 16, believes she was doped after going to her boyfriend's house last Saturday and says she woke up in a different house, surrounded by 30 men.
Hundreds of protesters on Friday demanded an end to sexual violence.
The video of the attack was put on social media, shocking Brazil.
Police are hunting more than 30 male suspects. Arrest warrants have been issued, including one for the boyfriend.
"I condemn most forcefully the rape of the teenager in Rio de Janeiro," Mr Temer said.
"It is absurd that in the 21st Century we should have to live with barbaric crimes such as this."
He said the authorities were working "to find those responsible" for the attack.
Rio police chief Fernando Veloso told a news conference that investigators will review forensic evidence.
"If these images hadn't been posted, maybe we wouldn't be here right now,'' he said, adding that many rapes go unreported.
The rape is said to have taken place in a poor community in western Rio over the weekend.
A 40-second-video was widely shared and followed by a wave of misogynistic comments, before the users' accounts were suspended.
In a message posted on Facebook, the victim said she was thankful for the support she had received and added: "I really thought I was going to be badly judged."
She later said: "All of us can go through this one day. It does not hurt the uterus but the soul because there are cruel people not being punished!! Thanks for the support."
The girl's grandmother told Brazilian media the family had watched the video and cried.
"I regretted watching it. When we heard the story we didn't believe what was happening. It's a great affliction. It's a depressing situation," she told Folha de S Paulo newspaper.
The assault has provoked an online campaign against what campaigners call a culture of rape in Brazil.
Experts say many cases of rape in Brazil go unreported as victims fear retaliation, shame, and blame for the violence they have suffered.
Hundreds of demonstrators gathered in Rio de Janeiro on Friday night, waving signs saying "Machismo Kills" and "No means no."
In Sao Paulo, protesters erected a mural with messages including "My body is not yours."
Dilma Rousseff, Brazil's first female president, who was impeached last month, expressed outrage over the Rio case by changing her profile picture on Facebook.
"Once more I reiterate my condemnation of violence against women," she wrote.
There has been an outpouring of anger on social media.
A collective of journalists has posted a satirical image of citizens donning devil's horns, condemning a rape victim for having provoked the attack.
The inscription reads "No to sexism", and the images, clockwise from top right: "But look at her clothes…"; "She deserved it!"; "16 years old and already has a son…"; "Apparently she was on drugs".
Brazil's interim President Michel Temer has called an emergency meeting of state security ministers after a gang rape of a teenage girl in Rio de Janeiro triggered wide condemnation.