Just over 3.7 million of an estimated seven million potential voters had added their names to the roll, a member of the independent election commission, Larbi Chouikha, told AFP ahead of the close of registration at midnight (2300 GMT).
The provisional figure, which does not include an estimated 700,000 to 800,000 Tunisians of voting age abroad, represented about 52 percent of potential voters still in the country.
The commission will release official figures on Tuesday. Registration opened on July 11 and was supposed to close on August 2, but was prolonged due to a slow turnout.
The October 23 election will be for a constituent assembly charged with drawing up a new constitution to replace that of the former dictatorial regime.