Glynis Sinclair, the deputy provost of Inverness who left the Lib Debs to join the SNP in 2011, complained to police that Provost Alex Graham – a LibDem councillor – had behaved aggressively towards her.
The alleged incident happened at a reception earlier this month in the Highland capital's Town House which was attended by Scottish Government ministers.
Ms Sinclair – the councillor for Culloden and Ardersier – said she had also made three separate complaints to senior figures within the Highland Council about his "bullying behaviour".
Ms Sinclair could not be contacted, but a spokesman for the Highland Council confirmed she had resigned.
A spokesman for Northern Constabulary said an inquiry into an alleged breach of the peace was still ongoing.