To describe the clinical features, histopathology and treatment of ocular surface squamous neoplasia (OSSN) with intraocular tumour extension. Retrospective study of 23 patients. The mean age at presentation of OSSN with intraocular tumour extension was 48 years. Mass (52%) and pain with redness (30%) were the most common presenting complaints. The mean duration of symptoms was 9 months. History of human immunodeficiency virus (HIV) infection was present in eight (35%) patients and one (4%) had xeroderma pigmentosum. History of prior tumour excision was noted in 16 (70%) patients. The mean basal diameter was 17 mm and the mean tumour thickness was 4 mm. Fifteen (65%) tumours had a nodulo-ulcerative tumour pattern at the time of detection of intraocular extension of OSSN. Anterior chamber cells and flare was noted in five (24%) cases and two (9%) patients had secondary glaucoma. Ultrasound biomicroscopy (UBM) (n = 11) revealed blunting of anterior chamber in three (27%) cases and uveal thickening in seven (67%) cases. Over the course of follow-up, extended enucleation (n = 6; 26%) or orbital exenteration (n = 17; 74%) was required for tumour control. At a mean follow-up period of 18 months, locoregional lymph node metastasis was seen in two (9%) patients, and one patient died with systemic metastasis. On histopathology, ciliary body was involved in all (100%) cases. Multiple tumour recurrences with history of prior tumour excision and nodulo-ulcerative tumour pattern are commonly associated with intraocular tumour extension of OSSN. UBM is a useful tool to detect intraocular extension of OSSN.