Loss-making HMD says it will sell 872,000 shares in the steel maker on the open market.
Analysts say the deal is part of the ongoing restructuring effort at the shipbuilder.
They note it does not have immediate cash flow issues but is freeing up capital that may be required for future projects.
The block sale has yet to be priced but the stock is worth $243m based on the present trading price of Posco.
