Castle Union was the code name given to one of the tests in the Operation Castle series of United States nuclear tests.

It differed from the Castle Romeo "Runt" device, tested shortly before, in using highly enriched lithium (approximately 95% lithium-6; natural lithium is a mixture of lithium-6 and lithium-7 isotopes).

The test took place on 26 April 1954 at Bikini atoll of the Marshall Islands, on a barge moored in the lagoon, off Yurochi island.

Like the Ivy Mike, Castle Bravo, and Castle Romeo tests, a large percentage of the yield was produced by fast fission of the natural uranium tamper, which contributed to the extensive fallout caused by these tests.

As the highly enriched lithium was both expensive and scarce at the time, it limited the number of these weapons that could be produced.