The £38m takeover deal will see the existing management team kept on at the Lanarkshire firm.
Motherwell Bridge employs 300 staff, mainly in the UK and boasts a strong international reputation stretching back over 100 years.
Many are employed in upkeeping gasholders and maintaining and refurbishing heat exchangers primarily in the UK continental shelf.
The firm’s project portfolio includes some of the most innovative projects in the world, including some of the world’s largest built storage tanks.
Last year it generated trading profits of £4.8m from £34.6m turnover.
Joe Oatley CEO of Cape said: “The acquisition of Motherwell Bridge is in line with Cape’s strategy to broaden our portfolio of critical industrial services.
“Motherwell Bridge has tremendous expertise, reputation and brand recognition in the oil and gas storage tank market, including a market leading position in the UK and we expect to accelerate Motherwell Bridge overseas growth through Cape’s international footprint.
“By combining the strengths of Cape and Motherwell Bridge we are uniquely able to provide a complete tank maintenance solution to clients in the oil and gas market.”