In a milestone for Downtown in the post-9/11 era, 7 World Trade Center — built by Larry Silverstein in the face of withering criticism and claims it would be a “white elephant” — is now 100 percent leased.

Financial firm MSCI, now based at 1 Chase Plaza, has leased 125,000 square feet in the tower’s top floors, sources told The Post.

The 20-year lease means that the 52-story, 1.7 million square-foot skyscraper across from the actual WTC site is now full except for a few tiny spaces almost too negligible to count.

The tower, built without any presigned tenants, opened in 2006. It gradually drew tenants paying the highest prices ever paid downtown — several above $70 a square foot. The building’s occupants include German bank WestLB and Moody’s.