Merkel cell carcinoma (MCC) is a rare and aggressive neuroendocrine tumor characterized with high rate of metastasis and mortality. It usually appears on head and neck as solitary red lump. We presented a unique case of collision tumor of MCC with invasive squamous cell carcinoma (SCC) that was incidentally revealed by an intraoperative consultation. A 52-year-old white man presented for wide excision of chest skin lesion after recent biopsy initially diagnosed as SCC in situ. The lesion was persistent over 20 years with recent increase in size. Physical examination revealed a 15 x 20 cm sternal exophytic crusty skin lesion with palpable adenopathies at right axillary and left periclavicular neck regions. Intraoperative frozen examination of metastatic lymph nodes revealed monotonous small basophilic tumor cells with scant cytoplasm, argued against conventional type of SCC. Initial examination of the skin excision specimen confirmed SCC in situ, which was again questioned by detecting 29 of 77 resected lymph nodes that showed neoplastic cells with MCC feature. These cells are positive for CK20, synaptophysin, and p63. An extensive skin lesion sampling was performed to reveal a collision tumor with small focus of MCC (1.3 cm) overlying invasive SCC (1.5 cm). This case presents microscopic findings of a rarely reported collision tumor within a widespread background of SCC in situ. Caution should be taken when MCC is suspected as the distinctive gross feature of MCC can be hidden by the extensive background of nodularity or ulceration of SCC.