The chain, which floated on the stock market in June 2014, reported like-for-like growth of 1.1% for the 13 weeks to 27 June.
That was markedly lower than the 6% recorded in the same period last year.
B&M blamed disappointing weather in May, which held back sales of outdoor seasonal products, for the decline.
Chief executive Simon Arora said: "As consumer spending improves, it is apparent that shoppers remain keen to spend with value retailers and we continue to win market share."
Discount retailers such as Poundland, 99p Stores and B&M have opened hundreds of stores in recent years and won customers from supermarket chains as shoppers seek lower prices.
B&M opened 25 net new stores in the quarter, with some of the new openings being among its most successful.
The company now has 450 outlets in the UK and plans to open a total of 80 this year as it heads towards a target of 850.
It also has 51 stores in Germany mostly under the "Jawoll" brand.
"We remain confident that future store expansion opportunities will allow us to increase market share in both the UK and Germany," B&M said.
Total revenue for the quarter rose 24.4% to Â£456.6m compared with the same period last year.
Shares rose 2.8%, or 9.7p, to 349.7p in morning trading in London, valuing the group at Â£3.5bn.
The shares have risen more than 22% since the float last year.
The flotation resulted in a Â£1bn windfall for the three Arora brothers, who have led the expansion of the company.

Burials were held for 33 of the 84 people whose bodies have been recovered. Several local people are still missing.
Thousands of people gathered in the square outside the town's main church to pay their respects in front of a long row of coffins.
Rescue workers are still searching the local river for victims.
One resident of Salgar particularly hard hit is the city's cemetery worker who has prepared 15 graves for members of his own family - mostly cousins who died in the flood.
A spokesman for the national emergency authorities said he expected to end the search for victims on Thursday or Friday.
He said between 20 and 25 local people caught in the flood had not been accounted for.
Locals and firefighters have been searching a 40km-long (25 mile) stretch of land on the banks of the Liboriana river.
Most of the residents were asleep when the landslide hit at 03:00 local time (08:00 GMT) on Monday.
The landslide was triggered by heavy rains that sent torrents of water and mud crashing into the La Liboriana ravine, causing an overflow that destroyed the neighbourhood below.
Sales growth at B&M, the discount retailer chaired by former Tesco chief executive Sir Terry Leahy, has slowed considerably in the UK.
[NEXT_CONCEPT]
A mass funeral has taken place in the Colombian town of Salgar for victims of a devastating mudslide on Monday.