HMS Phosphorus was the Dutch naval brig Haasje that the Royal Navy captured in 1803 and took into service in 1804 as a fireship.

Haasje was a Dutch naval brig that HMS Caroline captured on 2 August 1803 and sent into Saint Helena.

She was 30 hours out of the Cape of Good Hope with dispatches for Batavia (Dutch East Indies) when she encountered the British.

[6] At daybreak on 14 August 1806 Phosphorus was eight leagues south of the Isle of Wight on her way to join the Channel fleet.

[1] Phosphorus, Royalist, and Bloodhound shared in the prize money for the Danish ship Æolus, which they took on 19 August 1807, early in the Gunboat War between Britain and Denmark.

[8] Phosphorus shared with the hired armed cutter Active in the proceeds of the detention on 28 August of the Danish vessel Ferneijelsen.

[9] The "Principal Officers and Commissioners of His Majesty's Navy" offered Phosphorus, lying at Sheerness, for sale on 24 March 1810.

[12] The executors of the will of Thomas Outibridge Medley, owner of Phosphorus, advertised on 30 March 1813, for all creditors and debtors to respond.