It is not clear exactly what price the owner of Freuds paid for the 9.7 million shares yesterday.
However, the shares were trading between 38.4p and 45p on the day, making the transaction worth between £3.72m and £4.37m.
They closed today at 50p, leaving Freud with a paper profit of at least £485,000.
Huntsworth's share price had not been below 40p since late 2012, before it sold a 19.6 per cent stake to Chinese marketing services company BlueFocus as part of a strategic partnership in 2013.
The share price of the group, which also owns Huntsworth Health and Red, started to crumble after climbing above 70p in April. Investors took further fright when it issued a profit warning earlier this month.
It is the second major investment this week by Freud after the "significant minority" stake he took in Simon Kelner's PR agency Seven Dials alongside the agency's chairman Andrew McGuinness joining Freuds as chief executive.