The Lothian MSP had been with Edinburgh College lecturer Louise Riddell for nine years.
The couple got engaged on holiday on Mallorca in August last year and exchanged diamond-encrusted rings.
At the time Ms Dugdale said: “I’m utterly thrilled to be marrying the love of my life.”
They met at a party in the Capital in 2008, and bought a flat near the Scottish Parliament together in 2013.
It is understood the couple separated by mutual consent and no other person is involved.
A Labour spokesman said: “Kezia and Louise have separated and request that their privacy is respected.”