The sale will come as a welcome boost to HMV, which has already issued three profit warnings this year and saw its share price collapse after revealing it was on course to breach its banking covenants at the end of April.
Mamut is currently a 6.7% shareholder in HMV and the sale of Waterstone's will be conditional upon approval of shareholders, the Pensions Regulator and consent from HMV's lending banks.
In a statement to shareholders released by HMV today, the company said the sale of Waterstone's represented an important step towards strengthening the capital structure of the remaining HMV Group help the organisation to deliver the greatest value from the HMV UK and Live businesses.
HMV Group chief executive Simon Fox added, "We expect this deal to enable the group to achieve a reduction in the group's borrowing requirements, and, in turn, focus on plans for transforming the HMV Group into a broad-based entertainment business."
Mamut first began talks with the high street music retailer at the beginning of this year - and HMV said it hoped the sale of Waterstone's would be finalised by the end of June.