The Dysphagia Outcome and Severity Scale is used both clinically and within dysphagia research, internationally. Although it was developed using videofluoroscopic swallowing studies, it is frequently used to rate Flexible Endoscopic Evaluations of Swallowing. The validity and reliability of DOSS-use with FEES, however, has not previously been evaluated. This study investigated the validity and rater reliability of clinicians using DOSS to rate FEES. Eleven Speech-Language Pathologists (SLPs) with varied dysphagia experience were recruited to review and DOSS-rate 17 soundless FEES (198 bolus swallows) recorded from 11 heterogenic dysphagic patients (2 cases with repeat FEES) and 4 healthy adults. The SLPs DOSS-ratings were compared against the initial comprehensive dysphagia evaluation (including patient diagnosis, interview, cranial nerve and complete FEES assessment) with Functional Oral Intake Scale (FOIS) and DOSS outcome measures. The SLPs were blinded to patient details and comprehensive dysphagia examination. Re-randomised rating of FEES cases occurred two weeks later (intra rater reliability). Criterion validity for DOSS-ratings (compared against comprehensive dysphagia evaluation with FOIS and DOSS) were strong–very strong (rs= 0.858 and 0.936 respectively; p < 0.001). Inter rater reliability demonstrated high agreement (α = 0.891), also intra rater reliability demonstrated almost perfect agreement (Kw = 0.945). This study’s results, with strong–very strong criterion validity and high rater reliability by SLPs, adds to the evidence for DOSS-use with FEES. Future validity research comparing DOSS with both FEES and VFSS simultaneously is recommended.