PeO Wärring, editor-in-chief of the Eskilstuna Kuriren was take to hospital after he was knocked out cold after leaving the bar in the centre of the city in the early hours of Sunday morning.
"We have opened a preliminary investigation," said Lars Franzell at Sörmlands police.
According to a report in the Expressen daily, Wärring had pointed out to staff that a fellow customer bore a Hells Angels vest earlier in the evening.
Wärring left the bar shortly after and according to Eskilstuna Kuriren he was hit on the back of the head and sustained punches and kicks. He was then taken to hospital by ambulance.
According to the newspaper's publisher Elisabeth Bäck, Wärring had been discussing journalism and the newspaper's reporting in the bar. According to some reports the discussion had become heated.
The newspaper has previously received threats after writing about organized crime in the city although it remained unclear if the alleged assault is connected.
The police have called for witnesses and plan to conduct interviews on Monday.