Among the index heavyweights, H&M and Boliden fell by 5.2 and 3.9 percent, respectively.
Broadband operator Bahnhof's shares, listed on First North, surged 17.1 percent after a bid from Norway's Telenor.
US President Donald Trump said on Wednesday that he believes the ceasefire with Iran is over. Both Iran and the US have carried out intense attacks, and Trump is threatening further attacks.
The price of oil is moving upwards, with a barrel of North Sea Brent costing $79.60.
Leading European stock exchanges also fell, as did Wall Street, which started Wednesday in the red across all indexes.
The Stockholm Stock Exchange's broad OMXS index fell 0.8 percent on Tuesday. Since the turn of the year, the index has been up 3.8 percent.
