The company did not specify the amount it invested, but Ms Rinehart said it was "significant investment for Hancock" and the company was pleased to get an allotment during the oversubscribed IPO.
The Wall Street Journal reported the stake was more than $US1 billion ($1.4 billion).
Referring to SpaceX and Tesla, Australia's richest woman noted Mr Musk had built two of the world's 10-most valuable companies by market capitalisation, an extraordinary achievement.
"Elon has done what very few people in history have done," Ms Rinehart said.
"He has not just imagined the future, he has built companies capable of delivering it, and helped to keep American technology at the forefront."
The weekend's IPO raised the most funds in history, easily surpassing the $29.4 billion raised by Saudi Arabia's state-owned company in 2019.
It also made Mr Musk the world's first trillionaire, at least on paper, via his equity stakes in SpaceX and Tesla.
Ms Rinehart noted SpaceX was the first company to launch a liquid-fuel rocket to orbit in 2008, the first to dock a private spacecraft with the International Space Station in 2012, and the first, through Starlink, to begin deploying large-scale broadband satellite constellations in 2019.
"We see SpaceX as a rare business: led by a truly exceptional person, technically exceptional and operating in sectors that are crucial, and with long-term potential," Ms Rinehart said.
"Hancock favours investing in industries led by sensible, hard-working, patriotic and exceptional people. Elon excels in every regard."
Superannuation funds, institutional investors and tens of thousands of Australian retail investors took part in the float, which raised $US75 billion for SpaceX.
In its first day of trading on the Nasdaq on Friday, SpaceX shares closed up 19.2 per cent.
On Australian investing platform Stake, there was more than $US21 million in traded value on IPO day, more than 34 times the volume semiconductor company Cerebras experienced on its IPO day on May 14.
That one day of trading made SpaceX the most traded stock on Stake for June, overtaking NVIDIA, the company said.
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