Transport Company President Wuttichart Kalayanamitr said the deal was signed with Nattakan (Cambodia) Company, Cambodia's international bus firm.
The two routes link Bangkok-Aranyaprathet-Poipet-Siem Reap and Bangkok-Aranyaprathet-Poipet-Phnom Penh.
Mr Wuttichart said the route launch resulted from a bilateral meeting in May, agreeing to an Exchange of Traffic Rights at the Aranyaprathet-Poipet border crossing.
He said the first route from Bangkok to Siem Reap is 424km taking seven hours, to be operated twice daily at an initial fare of Bt750 (US$25).
The second route, from the Thai capital to Phnom Penh, takes 11 hours for the 719km. One trip will be operated daily at a fare of Bt900 (US$30).
The launch is expected boost tourism and economic activity between the two countries.
Mr Wuttichart said it is expected to draw at least as many as 40,000-50,000 passengers annually, a number which he expects to increase accordingly once the ASEAN economic community (AEC) takes effect in 2015.
Thailand will open four new bus routes to Cambodia next year, making six routes in total. It will also provide other bus routes connecting with other neighbouring countries by 2015.