According to the document, the firm will now not replace employees who have recently handed in their notice, or expand into new positions which were planned for in previous budgets.
This move follows the cutting of around 4,200 jobs since the beginning of 2008, Reuters reports.
Merrill has been hit with $45 billion of losses and writedowns relating to the credit crunch, which has put its capital ratios under considerable strain.
However, its chief executive John Thain suggested earlier this month that the bank would soon return to profitability.
According to the document, Merrill completed most of its new hirings over the first and second quarters of 2008.
It also claimed that the bank's retail brokerage unit - which currently has around 17,000 workers - would remain unaffected by the freeze.
At the end of June, Merrill retained around 60,000 employees in total.
The memo was originally sent out last Wednesday.