Summary This study reviews seven cases of focal peritarsal infection with long digital extensor tendon sheath (LoDETS) tenosynovitis, which presented to the same metropolitan equine practice over a 3‐year period (2016‐2019). All horses presented with acute‐onset severe hindlimb lameness, in combination with swelling and pain of the dorsal tarsus. In all cases, tarsal ultrasound demonstrated diffuse thickening of subcutaneous tissues and increased hypoechoic fluid volume in the LoDETS. Sampling and analysis of the LoDETS fluid demonstrated increased nucleated cell count, neutrophil percentage or total protein, consistent with sepsis in 6/7 cases. No bacterial organisms were identified on cytology or culture. Treatment consisted of systemic and local antimicrobial therapies, nonsteroidal anti‐inflammatory drugs and hand walking. Treatment duration ranged from 15 to 33 days. Lameness resolved in five cases; the remaining two cases were euthanised due to complicating factors prior to resolution of lameness. The study demonstrates the use of ultrasound and synovial fluid analysis in the diagnostic work‐up of suspected cases of focal peritarsal infection and identifies concurrent sepsis of the LoDETS as a novel finding. The prognosis for resolution of peritarsal cellulitis with concurrent sepsis of the LoDETS is considered good, with early diagnosis and appropriate antimicrobial treatment.