The company received commitments to place 166.67 million shares at 6c per share, a 10.4% discount to the last closing price.
Morgans acted as lead manager, with strong demand reported from existing and new institutional and retail investors.
Poseidon's two major shareholders, Andrew Forrest's Wyloo Metals and US-based Black Mountain Metals, did not participate in the placement.
The proceeds will be used to accelerate exploration at Black Swan to drill out the high-grade Golden Swan discovery and look for more nickel in the Southern Terrace.
Drilling last month on the Southern Terrace basal contact returned 6.4m at 9.6% nickel (true width 3.7m), including 1.6m at 14.89% (true width 0.9m).
Poseidon recently started a drill drive to provide a platform for resource definition drilling at Golden Swan.
Contractor WestAuz fired the first cut last week, with the drive expected to be completed in February 2021.
"All these activities are aimed at building a sufficient high-grade inventory at Black Swan to allow mining to recommence after a 10-year hiatus," Poseidon managing director Peter Harold said.
"We are very excited about the potential of the Southern Terrace and thank Morgans for their assistance with this placement."
The raising will bolster Poseidon's cash position to about $22 million.
Poseidon shares as much as doubled in 2020, but have pulled back from the October high of 9.7c.
Shares last traded 1.5% lower at 6.6c, valuing the company at about $167 million.
