T1 melanomas, despite their favorable prognosis, account for 25% of melanoma deaths. The American Joint Committee on Cancer (AJCC) 7th edition melanoma staging, implemented in 2010, replaced the level of invasion with the mitotic rate for T1 substaging, on the basis of prognostic modeling, not prediction of occult lymph node metastasis. Previously, sentinel lymph node biopsy (SLNB) was recommended for T1b patients, whereas current guidelines suggest SLNB for select high-risk T1 melanomas. We investigated the effect of this staging change on the performance and outcomes of SLNB for T1 melanoma. Using 2004–2010 data from the Surveillance, Epidemiology, and End Results (SEER) Registry, we identified 32 527 cases of T1 melanoma and compared pre-2010 (N=27 170) with 2010 (N=5357) data. We used χ2-tests, t-tests, and logistic regression models for analysis. After implementation of the 2010 AJCC staging system, SLNB for T1 patients increased from 12.1% (2004) to 14.4% (2010), despite a decrease for T1b melanomas (40.9 to 33.3%; both P values<0.001), and there was no change in SLNB for melanomas that were 0.7 mm or thicker (38.3 and 39.3%). T-stage, thickness, level, ulceration, age, and geographic region were correlated with SLNB performance (all P values<0.001). For T1 patients, overall SLN positivity rates were 6.1% pre-2010 and 7.8% in 2010 (P=0.12), while nearly doubling for T1a patients (3.6 to 6.6%, P=0.03). SLN-positive patients had diminished cancer-specific survival (P<0.001). SLNB for T1b melanomas decreased after AJCC T1 reclassification, without changing for melanomas that were 0.7 mm or thicker. SLN positivity rates increased for T1a melanomas, and SLN status was prognostic for T1 patients. Improved strategies to identify high-risk T1 melanoma patients most likely to benefit from SLN surgery and to minimize clinical practice variation would be valuable.