The Poseidon bubble was a stock market bubble in which the price of Australian mining shares soared in late 1969, then crashed in early 1970. It was triggered by the Poseidon NL company's discovery of a promising site for nickel mining in September 1969.

In the late 1960s, nickel was in high demand due to the Vietnam War, but there was a shortage of supply due to industrial action against the major Canadian supplier Inco. These factors pushed the price of nickel to record levels, peaking at around £7,000 on the London market early in November 1969. In September 1969, the mining company Poseidon NL made a major nickel discovery at Windarra in the Shire of Laverton, Western Australia. In early September their shares had been trading at $0.80, but as information about the discovery was released, the price rose until it was trading at $12.30 on October 1. After this, very little further information came to light, but the price continued to climb due to speculation; at one point, a UK broker suggested a value of up to $382 a share.

The price of Poseidon shares quickly became too high for many investors, so some investors turned to other nickel stocks, stocks in other mines near Windarra, and eventually other mining stocks in general. As the price of mining shares grew, numerous new companies were listed by promoters looking to cash in. Some of these new listings did not even have any mining leases, let along viable mines. Many investors lost money on these shady listings, and this attracted substantial negative press. Thus the image of mining stocks was tainted, and the prices began to fall. Mining stocks peaked in January 1970, then immediately crashed. Poseidon shares peaked in February 1970, and fell rapidly thereafter.

By the time Poseidon actually started producing nickel, the price of nickel had fallen. Also, the nickel ore was of a lower grade than originally thought, so extraction costs were higher. Profits from the mine were not sufficient to keep Poseidon afloat, and in 1976 it delisted. Western Mining then took over the mine, operating it until 1991.

In 1974, The Rae Committee handed down its report on the Poseidon bubble, in which it documented numerous cases of improper trade practices. It recommended a number of changes to the regulation of stock markets, which ultimately led to Australia's national companies and securities legislation.

In the late 1980s Robert Champion de Crespigny took over the Poseidon Company and it became part of Normandy Mining, the largest gold miner in Australia. In 2001 Normandy Mining, including Poseidon, was taken over by the Newmont Mining Corporation, now the world's largest producer of gold.