USS Humboldt (AVP-21) was a United States Navy Barnegat-class small seaplane tender in commission from 1941 to 1947 that served in the Atlantic during World War II.

After stops at San Juan, Puerto Rico, and Trinidad, she arrived at Recife, Brazil, on 5 August 1942 and began tending the seaplanes of Patrol Squadron 83 (VP-83).

Humboldt supplied and repaired seaplanes and, in addition, carried aviation gasoline to outlying air bases along the Brazilian coast while engaging in antisubmarine patrols herself.

This meeting helped to achieve even closer cooperation between the naval units of the two countries and involved discussions of the ongoing support and role of Brazil in World War II.

Departing on 23 August 1943, she carried supplies and parts to U.S. Navy fleet air wings in Newfoundland, Iceland, and the United Kingdom.

Humboldt was soon underway again, this time to bring an experienced U.S. Navy submarine officer to rendezvous with the escort aircraft carrier USS Guadalcanal hunter-killer group, which had just captured the U-505 in an epic encounter on 4 June 1944.

Reclassified as a miscellaneous auxiliary and redesignated AG-121 on 30 July 1945, Humboldt was to serve as a broadcast and teletype center for correspondents during the planned invasion of Japan in 1945–1946.

The Navy struck her from the Naval Vessel Register in 1970 and sold her for scrapping to Cantieri Navali, Genoa, Italy, for a bid amount of $60,000 (USD).