Saks Fifth Avenue and Neiman Marcus have reportedly agreed to a blockbuster merger that combines the nation’s two dominant luxury retailers — and Amazon is joining the deal, too.

Saks owner HBC, whose talks to merge with its longtime a were first reported by The Post last summer, will acquire Neiman Marcus for $2.65 billion, according to sources close to the situation — a once unthinkable tie-up that comes as both chains grappled with a multiyear slowdown in luxury spending.

In a surprise twist, Seattle-based Amazon — the e-tailing giant headed by billionaire Jeff Bezos — will become not only a minority investor in the new company — to be called Saks Global — but also will provide “technology and logistics services” to the merged operation, sources told The Post.