Ayman Nofal escaped last week during the anti-government unrest gripping Egypt and made his way into the sealed Gaza Strip through a smuggling tunnel under the border on Saturday. He was greeted by a celebratory crowd, some of whom fired shots in the air.
Hamas says Nofal was arrested in 2008 in Egypt but denies he was plotting bomb attacks there.
A Hamas official did not say how Nofal escaped. He spoke on condition of anonymity due to the sensitivity of the issue.
Egypt's government is wary of the Islamic militants of Hamas, having battled for years against local militants.