Six soldiers from Niger's army were killed in an ambush by Boko Haram militants, the country's interior ministry said.
The attack happened near the town of Diffa close to the border with Nigeria early on Wednesday.
It comes two weeks after a Boko Haram assault on a military convoy in the same area that left one soldier dead.
The Islamist group is based in Nigeria but is being tackled by a multinational force, including soldiers from Niger.
In the latest attack, three other soldiers were wounded. Niger's military deployed aircraft to try and track those responsible soon afterwards.
Niger's President Mahamadou Issoufou has been praised in the West for rallying neighbouring countries to fight Boko Haram.
On Wednesday, the country's constitutional court confirmed the result of the second round of presidential elections, which saw Mr Issoufou re-elected with more than 92% of the vote, a result the opposition dismissed.
Mr Issoufou will be sworn in on Saturday.
Boko Haram at a glance:
- Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language
- Launched military operations in 2009
- Thousands killed, mostly in north-eastern Nigeria, hundreds abducted, including at least 200 schoolgirls
- Joined so-called Islamic State, now calls itself IS's "West African province"
- Seized large area in north-east, where it declared caliphate
- Regional force has retaken most territory last year