Police described the bomber, who struck in the city of Mardan, as a male teenager dressed in school uniform.
He attacked as recruits conducted morning exercises.
Correspondents say there has been a relative lull in the number of attacks on the army in the region, where Taliban militants are active.
A Taliban spokesman said the militants had carried out the bombing.
Prime Minister Yusuf Raza Gilani condemned the attack on the Punjab Regiment Centre.
"Such cowardly attacks cannot affect the morale of the security agencies and the resolve of the nation to eradicate terrorism," he said in a statement.
Mardan police official Abdullah Khan told the BBC that the teenage bomber had been wearing the uniform of a school located in the area, Aziz Bhatti College.
The bomber struck at 0800 (0300 GMT) as recruits were doing physical training on the parade ground, he said.
A bomb attack on the same centre in 2006 killed at least 20 soldiers.
The Punjab Regiment is one of Pakistan's most famous military units.
There has been an increasing use of teenage bombers across Pakistan, the BBC's Aleem Maqbool reports.
Pakistani troops have found whole camps in the north-west where children as young as 10 or 12 were being trained to become suicide attackers, our correspondent says.
Speaking to AFP news agency from an undisclosed location, Taliban spokesman Azam Tariq said the organisation carried out Thursday's attack, and was proud to avenge people killed by US drone attacks and by military operations in the tribal areas.
The Taliban, he said, would continue to carry out attacks on "those who protect the Americans".