St. Patrick's Co-Cathedral, Gothic Revival cathedral in Billings, United States.

St. Patrick's Co-Cathedral is a Gothic Revival church built in 1904 in Billings, Montana, and serves with St. Ann's Cathedral in Great Falls as a seat of the Diocese of Great Falls-Billings. Its interior was reworked in 1954 with Byzantine-style decorative painting and a new rose window above the choir loft, and later changes removed the side altars and communion rail.

The cathedral was built in 1904 and grew from the first Catholic parish in Billings, which was established as St. Joachim in 1887. This early founding shows how the Catholic community developed as Billings expanded.

The parish began as St. Joachim in 1887; the present church was built in 1904 and became a co-cathedral on February 14, 1980, when Pope John Paul II renamed the Diocese of Great Falls as the Diocese of Great Falls-Billings. It shares the seat with St. Ann's Cathedral in Great Falls.

The cathedral stands at 215 N 31st Street in Billings, near the downtown core, and is shared as a seat with St. Ann's Cathedral in Great Falls. The 1990s work added a new gathering space and sacristies, so entry is through that front gathering area rather than the 1904 doors alone.

A major renovation in 1954 brought Byzantine-style interior paintings and a new rose window to the space, blending different architectural styles. This combination of older Gothic elements with newer Byzantine details makes the cathedral architecturally distinctive.