The House flag of Donaldson Line Ltd, Glasgow. A red, white and blue tricolour with a black 'D' on a central white stripe. The flag is made of a wool and synthetic fibre bunting. It has a cotton hoist and is machine sewn. A rope and toggle is attached.

The company was acquired by the Ulster Steam Ship Co. in 1967 and after that date their ships flew the Donaldson flag below their own. The remaining Donaldson vessel, 'Santona', flew the Donaldson flag above the Ulster Steam Ship Company one and retained the Donaldson funnel.

The company was founded by William Falconer Donaldson (1830-1890) and John Donaldson (1833-1889). Initially trading as shipbrokers, charterers and insurance agents, they bought their first vessel in 1858 and operated a route between Glasgow and the River Plate. In the 1870's they began sailings to Canada in the summer and South America in the winter. In the 1880's they started the carriage of live cattle on the North Atlantic and in 1905 entered the Canadian passenger trade.

Donaldson Line Ltd was set up in 1913 to avoid death duties but remained a family firm. In 1919 Anchor-Donaldson was formed, Anchor Line being at the time a Cunard subsidiary. Donaldson's South American Line Ltd was founded in the same year, being eventually wound up in 1941. When Anchor Line went into liquidation in 1935 as a result of a slump in business during the depression, Donaldson's bought out their shares and changed the name to Donaldson Atlantic Line. The company suffered in the post-war period due to competition from passenger air travel and the costs of containerization. The company went into liquidation in 1970 with the name and goodwill being purchased by the Ulster Steamship Company.