Scotland in Union published an interactive website for voters whose priority is preventing an SNP majority and second referendum on independence.
It suggests voters back their favourite established pro-UK party in the regional party list vote, either Labour, the Tories or the Lib Dems.
It also warns against splitting the pro-UK vote by supporting any other smaller party.
A Scotland in Union spokesperson said:“Polls are clear that a second referendum is not a priority for the people of Scotland and would divide our country.
“But the SNP will use every vote it receives to argue for another referendum on its negative vision to leave the UK.
“If you do not want your ballot paper at this election to be used to divide communities, friends and families, you should consider tactically voting to stop the SNP in your constituency, and voting for one of the three main pro-UK parties.
“Scotland in Union has received requests from supporters for advice on who to vote for to stop an SNP majority.
“Tactical voting could make the difference.
“So, if your priority in this election is anything other than a divisive referendum, then this guide will help you decide which parties to back with your two votes.”
The new campaign website allows everyone in Scotland to enter their postcode and receive a suggestion for their two votes.
The link will be sent to Scotland in Union’s 34,000 registered supporters as well as shared on social media.