The SNP reached a high of 120,000 in July last year, but it slipped to 118,000 last month.
The figures reported by the Herald, were not publicised by the party and were uncovered within a House of Commons briefing paper.
The Nationalist movement remains strong in Scotland and the SNP still have more members than all the other Scottish Parliament parties put together.
After the snap election earlier this year the Tories claimed that “peak Nat” had passed and Nicola Sturgeon’s party was on the wane.
The SNP’s membership grew exponentially in the wake of the No vote of 2014 but by December 2016, membership was down to 118,959, and it has kept falling.
The Commons paper states: “There are around 118,000 members of the Scottish National Party, as of August 2017, according to information from the party’s Central Office.
“This was a slight decrease compared to almost 119,000 members in December 2016.”
Political parties have no obligation to publish membership figures.
The SNP dared the other parties to publish their own membership numbers.
The slide coincides with Nicola Sturgeon calling a second independence referendum in March, and the party losing a third of its MPs in the June election, its worst reverse in 40 years.
The SNP’s firm pro-EU stance ahead of the snap general election in June was also a significant factor in the party failing to repeat its success at the previous poll in 2015, a report at the time found.
Meanwhile, Scottish Secretary David Mundell has said the question of independence needs to be “decoupled” from Brexi.
Mr Mundell claimed the two issues had been deliberately conflated in order to help the push for a second independence vote. But during a trade trip to South America, he called on the UK and Scottish governments to move on from “the issue that divided our country so badly” and work closer together on Brexit.