The Three Sisters are three similar self-anchored suspension bridges spanning the Allegheny River in downtown Pittsburgh, Pennsylvania at 6th, 7th, and 9th streets, generally running north–south.

The Three Sisters are historically significant because they are the only trio of nearly identical bridges, as well as the first self-anchored suspension spans, built in the United States.

County engineers successfully overcame challenges presented by federally mandated clearances, aesthetic and financial considerations raised by local agencies, and the lack of adequate anchorage points along the river banks.

The design’s solid plate deck-stiffening girder provided compressive support while lowering visual barriers between Pittsburgh and the historically distinct North side (formerly Allegheny City), annexed in 1907 in a contentious fight.

Though a few unstiffened suspension bridges exist, a longitudinal stiffening truss or girder is usually added to prevent excessive movement of the deck.

Because each of Pittsburgh's Three Sisters appears to be a self-contained unit not dependent on the river banks for anchorage, a debate ensued among engineers whether these structures were cantilevers rather than suspension bridges.

Each bridge was built in halves toward the center, with temporary diagonal struts placed between chain and deck to provide shear resistance.

The bridges were quite profitable, with several of the companies in the area paying 15% dividends yearly to stock holders, even with tolls at 1 cent per man and free for women.

In 1924, voters finally approved a $29.2 million bond issue at the county level to improve bridges and municipal structures.

Expecting a rubber stamp, contracts were let, but to everyone's shock the commission vetoed the designs as unaesthetic, preferring suspension bridges.