Jundullah announced its backing after meeting a three-man delegation representing IS led by al Zubair al Kuwaiti, the group's spokesperson Fahad Marwat told Reuters.
The group is one of several Pakistani groups exploring relations with IS, whose fighters have captured swathes of Iraq and Syria in a drive to set up a self-declared caliphate.
Analysts say that so far IS has mainly attracted sectarian groups rather than anti-state militants like the Taliban.
"They (Islamic State) are our brothers, whatever plan they have we will support them," said Jundullah spokesperson Marwat.
His comments follow the release of a video last month by five Pakistani Taliban commanders pledging support to IS. Islamic State also has contacts with the banned Pakistani group Lashkar-e-Jhangvi (LeJ), militants and security officials said.
"The top leadership of LeJ visited Saudi Arabia and met Islamic State leaders at an undisclosed location at Saudi-Syria border," one militant told Reuters.
He said the meeting took place more than a year ago.
Many Pakistani militants said they felt torn by loyalty to Afghan Taliban leader Mullah Omar, who has strong historical ties to al Qaeda. IS itself broke away from al Qaeda.
"All anti-Shia groups in Pakistan will welcome and support IS in Pakistan, though most of them will not announce it openly due to their allegiance to Mullah Omar," one said.
Jundullah has taken part in attacks on Shias in addition to attacks on members of other religions including the All Saints church bombing in Peshawar last year that left around 80 Christians dead.
Last week, a leaked government memo warned that IS had recruited 10,000-12,000 fighters inside Pakistan, but government insisted that the group had no presence there. Militants and security officials also dismissed the memo as not credible.
Several militants said that hundreds of Pakistanis have gone to fight in Syria, but had done so through the Taliban or on their own.
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It should not come as a surprise that one terrorist group is voicing its support for another terrorist group. Their common agenda is reflected through their terrorist activities on a regular basis. We’ve seen over and over that they will cross all lines to spread fear and chaos among people. The indiscriminate killings, kidnappings, and suicide attacks are executed on a regular basis. Our nations have simply refused to bow down to their acts of terror, and our shared will and desire to defeat terrorism motivates us to push ahead with our peace mission. It is important for us to remain unified against those who threaten the safety of our land and people. Our leaders and officials from the highest level continue to meet regularly to address our shared concerns in the region, and we hope to negate the common threat of terrorism through shared cooperation and assistance.
Ali Khan Digital Engagement Team, USCENTCOM
Hired killers with different name but master is one.Muslim countries leaders know the truth but no gutts to save own people.
"Undisclosed location on Saudi-Syria Border." This a such a big lie. Saudi Arabia and Syria don't share a common border. They are close to 200 KM apart from each other.
Pakistan Army keep burying them. That's the only solution. Whole nation is with Pakistan Armed forces.
These are the last breaths of evil beasts before their complete extinction on our soil InshAllah
They will end up same as did those before them. Pakistan Army Zindabad