Tyngsborough Bridge, Steel tied-arch bridge in Tyngsborough, Massachusetts, US.

Tyngsborough Bridge is a steel arch bridge spanning the Merrimack River in Massachusetts with a main arch section and supporting girders. The structure crosses the waterway with a single large opening and carries the local road network connecting the towns on either side.

The Tyngsborough Bridge was built in 1931 to replace a wooden Whipple truss bridge, sited alongside but not parallel to the old crossing. Rebuilt from 1975 amid cost controversy, it closed in November 2005 for structural repairs; a temporary Mabey Panel Bridge carried traffic while work began in 2009. Contractor S & R Corp. finished in 2012 at $19 million, after steel girders failed stress tests and lead paint had to be contained.

The bridge's open, braced rib design links it to Boston University Bridge, and its Pratt-type trussing and centre hinge are visible details. Route 113 traffic still crosses the 547-foot span, the longest of any steel rib through arch in Massachusetts.

Route 113 crosses the bridge between Tyngsborough and the west bank of the Merrimack River. During the 2005 closure a temporary Mabey Panel Bridge was erected alongside for traffic, and the main span reopened in 2012 after three years of repairs. Check local road notices before crossing while maintenance work is under way.

The 2009–2012 repairs cost $19 million rather than the contracted $16.4 million, partly because steel girders failed stress tests and had to be re-engineered. Workers also had to contain lead paint chips from the original 1931 structure so they would not fall into the Merrimack River.