The Nationalists issued their statement as the Order, which claims to have 50,000 members, said it would lay down party allegiances even if it meant "getting into bed with Labour", in order to beat the SNP.
Members will be urged to back whichever unionist party has the best chance of beating the SNP in individual areas.
The Order's Grand Master Ian Wilson said the Order had to prioritise the saving of the Union above all other considerations.
"There is no question in my mind that the biggest problem facing Scotland at the moment is the growth of Scottish nationalism. And the order – as one of Scotland's biggest unionist organisations – has got to get real about protecting the Union.
He added: "The reality is that the only party you can do that through in Scotland is the Labour Party."
In response, an SNP spokesman yesterday said that "everyone can express their view" as part of the debate over the future of the country.
David Kerr, the party's candidate for the forthcoming Glasgow North-East by-election, added: "There is a national conversation and all groups are very welcome and that includes those in the Orange Lodge. I look forward to having that debate."
Michael MacMahon, the Labour MSP for Hamilton North and Bellshill, and a Catholic, said that he welcomed the support. He said: "I have a good relationship with the Orange Order. They understand the importance of the Union and they understand the threat."