Despite the richness of the Diptera (true flies) and their environmental, medical and agricultural importance they are poorly studied in Saudi Arabia. Insect specimens were collected mainly using sweeping and Malaise traps, at 10 sites in southwest Saudi Arabia in 2012-2014 and 2017. Thirty nine species of Diptera belonging to 14 families were monitored, 20 of them were first time recorded in southwest Saudi Arabia namely; Ophiomyia arabica Deeming, Phytoliriomyza sp. n. near hispanica Spencer (Agromyzidae), Ommatius tenellus Van der Wulp (Asilidae), Katacamilla cavernicola Papp, Katacamilla ctenidia Barraclough, Katacamilla procavia Barraclough (Camillidae), Lasiambia sp. near albidipennis (Strobl), Rhopalopterum sp. (Chloropidae), Somatiosoma eremicolum Ebejer (Chyromidae), Conops (Conops) tomentosus Kröber, Physocephala antiqua (Wiedemann), Thecophora atra (Fabricius) (Conopidae), Platypalpus albiseta (Panze), Platypalpus flavicornis (Meigen), Platypalpus pachycera (Collin) (Hybotidae), Leptometopa coquillettiHendel (Milichidae), Efflatouniella vanharteni Hauser, Phycus rufofemoratus Kröeber, Ruppellia semiflava Wiedemann (Therevidae). This study has added new Diptera records which will provide the basis for future works on Diptera. We suggest that many more species will be revealed in the country, if new provinces and localities are investigated with the use of mass trapping methods.