USS Chiron (AGP-18) was a Portunus-class motor torpedo boat tender which saw brief service with the United States Navy during and just after World War II. She then served as the Argentinian merchant ship MV Altamar until she sank in 1960.

Authorized as the LST-542-class tank landing ship USS LST-1133, the ship was reclassified as a motor torpedo boat tender, assigned the hull number AGP-18, and renamed USS Chiron on 14 August 1944. Chiron was laid down by the Chicago Bridge and Iron Company at Seneca, Illinois, on 16 December 1944, launched on 10 March 1945, sponsored by Mrs. T. S. Tillman, and placed in reduced commission on 23 March 1945 for transit to her conversion yard. On 17 April 1945, she was decommissioned for her conversion into a motor torpedo boat tender. Her conversion took place at the Maryland Drydock Company at Baltimore, Maryland, and after its completion, she was recommissioned USS Chiron (AGP-18).

Chiron departed Norfolk, Virginia, on 1 November 1945 bound for Miami, Florida, where she arrived on 4 November 1945 to service the PT boats of Motor Torpedo Squadron 42 until 8 December 1945. On 14 December 1945 she got underway for New York City, where she was decommissioned on 20 February 1946 after only five months and 27 days in naval service. She was stricken from the Naval Vessel Register on 28 March 1946.

On 19 May 1947, Chiron was sold to commercial interests in Argentina. By 1948 she was serving as a merchant ship under the Argentinian flag with the name MV Altamar. She sank on 30 March 1960 during a voyage from Cabedelo to Belém, Brazil, with a cargo of grain. Her wreck was found in the Atlantic Ocean off the northeastern coast of Brazil on Manoel Luis Reef at 00°46′S 044°20′W / 0.767°S 44.333°W / -0.767; -44.333.

During her U.S. Navy service, Chiron received the following awards:

• American Campaign Medal
• World War II Victory Medal

 This article incorporates text from the public domain Dictionary of American Naval Fighting Ships. The entries can be found here and here.

• Ship details at Navsource.org
• Details at Shipscribe.com
• Wrecksite.eu information
• Naval Museum article
• Ibiblio.org site