Their bans will mean they will both miss next year's World Cup.
Akhtar, 31, and Asif, 23, were sent home in disgrace from the Champions Trophy in India on October 16 after the PCB said the performance-enhancing drug nandrolone had appeared in their samples.
The pair, who both have the right to an appeal, have insisted they did not take any banned substances knowingly, but still waived their rights to their 'B' sample testing.
The scandal is international cricket's first involving performance-enhancing drugs and Akhtar's ban is the sport's heaviest ever for doping.
"We gave a full chance to both the pacemen to fight their cases and after a thorough inquiry we feel they failed to prove their innocence," tribunal chief Shahid Hamid said in Lahore after hours of deliberations.
"The process of doping was investigated carefully. We made sure the tests were conducted properly, samples reached the laboratory in Malaysia safely and there was no error in testing in the laboratory.
"Both the players were satisfied and accepted the tests and gave their point of view but after a thorough inquiry and bound by the WADA (World Anti-Doping Agency) and the International Cricket Council (ICC) we reached the decision," said Hamid, a former governor of Punjab province.
"The two players are banned from representing Pakistan in any international match, cannot play in a domestic match under the auspices of the PCB, cannot take any monetary gains and cannot hold any official posts."
The pair can appeal the decision either with the same tribunal within seven days; with a different committee appointed by the PCB within 30 days; or with the sports court of arbitration in Lausanne, Switzerland.
Pakistan coach Bob Woolmer, physiotherapist Darryn Lifson and trainer Murray Stevenson appeared before the tribunal on Wednesday to give evidence about how the bowlers tested positive.
"Akhtar contested the case saying that he has been on a high-protein diet which contained beef, chicken meat and other dietary supplements and he also said he took some herbal medicines from a Hakim (herbal expert) but he couldn't prove them," Hamid added.
"Asif's case was different.... Pakistan physician Darryn Lifson confirmed he stopped him from taking a banned substance a few months back," said Hamid.
Not good news
Woolmer said the ban was "not good news for Pakistan."
"I was questioned about Asif and Akhtar for well over 40 minutes on Wednesday... but as far as the doping is concerned I don't have much to say," said Woolmer, who recommended the tests on 19 Pakistan players last month.
The ban may effectively end Akhtar's mercurial career, which has been marked by controversy over his bowling action controversy, indiscipline, injuries and ball tampering.
It is cricket's highest profile doping case since Australia's legendary leg-spinner Shane Warne was banned for a year in 2003 after testing positive for a diuretic.
Akhtar, who has taken 165 wickets in 42 Tests and 133 in 208 one-day internationals, is famously known as the "Rawalpindi Express" for his pace and for his home town.
He launched the fastest electronically measured ball ever bowled, at a speed of 100.23mph (161.3kmh), against England in February 2003 at the World Cup in South Africa.
Asif, whose career took off early this year before being held up by injuries, has 30 wickets in six Tests and 19 in 17 limited overs international.
The bans cap a bitter few months for Pakistan since they forfeited a Test match for the first time in history at the Oval in August in a row over ball-tampering.
Captain Inzamam-ul-Haq was banned for four one-dayers for bringing the game into disrepute through his actions at the Oval, and there have been musical chairs in both the captaincy and the PCB chairmanship since.