Ameritrade shares climbed 8.7%, while those of Charles Schwab rose 5.4% before the bell.
Charles Schwab last November agreed to buy TD Ameritrade in an all-stock deal valued at $26 billion at that time. The merger could create a brokerage giant in a market that has been ravaged by price wars.
The companies expect the deal to close in the second half of the year, and anticipate the integration to be completed between 18 and 36 months following the close.
Reporting by C Nivedita in New Delhi; Editing by Sriraj Kalluvila