Diagnosis of COVID19 is based on reverse transcription-polymerase chain reaction (RT-PCR) testing of nasopharyngeal swab (NPS) samples. As NPS sampling is a discomforting invasive procedure, studies were conducted to investigate the performance of saliva and mouthwash (MW) samples in RT-PCR. There is limited data about MW samples. We have developed a new method for concentrating gargle and mouthwash (GMW) samples to be used in RT-PCR. In our study, we aimed to investigate the performance of concentration of GMW samples in detection of SARS-CoV-2. A paired sample of NPS and GMW samples were collected from patients in 11 centers in Turkey. MW samples were concentrated using MyMagiCon-RW100® (Bio-T, Istanbul, Turkey). NPS, GMW and concentrated GMW samples were tested by RT-PCR for the presence of SARS-CoV2 and the results were compared. The viral RNA was detected in 47.5% of NPS samples, in 28.8% of GMW before concentration and in 37.5% GMW samples after concentration. Concentration of samples increased the number of samples in which SARS-CoV2 RNA was detected by 16.6%. The RT-PCR of concentrated GMW samples yielded better results than the RT-PCR of NPS samples in two centers. Concentrated GMW sampling can be an alternative method to NPS sampling in rapid and accurate diagnosis of COVID-19.