Khaled Meshaal told a rally in the Syrian capital Damascus his group was preparing for a new round of conflict.
The truce begun in February was based on Israel ending its attacks on Palestinians and releasing prisoners.
Hamas had already said it would pull out of the truce when Israel killed a military leader in November.
Addressing the crowd at the Yarmouk refugee camp near Damascus, Mr Meshaal said there was no room for a truce.
Mr Meshaal was named the organisation’s most senior figure after the assassination of Hamas leader Sheikh Ahmed Yassin in March 2004.