Enthesitis is a key pathology in spondyloarthritis (SpA), but diagnosis may be clinically challenging. The objective of this study was to investigate the prevalence of ultrasound enthesitis lesions in tender entheses in the heel region in patients with peripheral SpA. In 27 patients with tenderness upon palpation at the Achilles tendon or the plantar fascia insertion, ultrasound assessment of the affected enthesis was performed using greyscale and color Doppler mode. Images were evaluated using the Outcome Measures in Rheumatology (OMERACT) scoring system for enthesitis, scoring presence/absence of hypoechogenicity, thickening, calcifications/enthesophytes, and erosions, and color Doppler activity semi quantitatively from 0 to 3. A total enthesitis sum score was calculated. A second examiner scanned 10 patients for inter-reader reliability. Ultrasound signs of inflammatory enthesitis (thickening/hypoechogenicity and/or Doppler activity) were found in 48%, and 19% showed Doppler activity-all in the Achilles enthesis. Inflammatory pathologies other than enthesitis (e.g., tendinitis, arthritis, bursitis) were identified in 26% of tender heels. The ultrasound OMERACT scoring system for enthesitis lesions showed excellent intra- and inter-reader agreement in a clinical setting. In conclusion, less than 50% of clinically tender entheses are related to inflammatory enthesitis when assessed by ultrasound. Ultrasound is useful for diagnosing other pathologies that may explain tenderness in the area.