Sirisena's office said Arundika Fernando, junior minister for tourism and Christian affairs, was expelled from the government under the executive powers of the president.
The brief statement did not give a reason for his dismissal, but official sources said the move prevented Fernando from engineering defections to a breakaway faction of Sirisena's Sri Lanka Freedom Party.
"With this sacking the president has asserted his authority and sent a signal he won't hesitate to expel more," a source close to the presidency said, citing Fernando's previous assertion that around a dozen ministers were planning to leave the government.
Fernando was not immediately available for comment, but he told the Lankadeepa daily website that his sacking was unlikely to discourage other dissidents in the government.
Sirisena's party, the junior partner in Sri Lanka's coalition since August 2015, is already split between him and former strongman president Mahinda Rajapakse, who is an MP and has considerable support within the party.
It is not yet clear whether the potential new faction was trying to join the side of the former president or remain as a third faction within the party.
Three weeks ago Sirisena sacked justice minister Wijeyadasa Rajapakshe, who publicly denounced the government's $1.1 billion sale in July of a 70 percent stake in a port to state-owned China Merchants Port Holdings.
Sirisena and his Prime Minister Ranil Wickremesinghe have vowed to continue their power-sharing arrangement until 2020 when the next general election is due.
However, there are reports of squabbles within the coalition and many fiscal policy measures have been either toned down or completely withdrawn in recent months due to infighting.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)