The group, Jundullah, is one of the dreaded factions which parted ways with Taliban who are being targeted by Pakistan security forces in the restive tribal areas.
The decision to align with the IS was taken after its three-member delegation led by Zubair al-Kuwaiti visited Jundullah, a militant leader Fahad Marwat said.
"They (Islamic State) are our brothers, whatever they plan we will support them," Marwat said.
The decision by Jundullah came after a campaign of graffiti and hoisting of IS flags in support of the group in the country.
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Pakistan made some IS related arrests in Multan and Lahore last week, according to a security official.
A secret report by security agencies leaked to media warned that more than 10,000 people have been recruited by the IS in a drive to make inroads in Pakistan.
Officials have so far not shown any major concerns at IS efforts to woo Pakistanis.
They believe that militants are using its name to get attention after successful military operation by army which killed over 1,200 rebels since June.
It also claimed responsibility for the Wagah border bombing which killed around 60 people.