Tumor-to-tumor metastasis is an exceptionally rare phenomenon, even among cases with synchronous primary malignant neoplasms. It is defined as the metastasis of one primary malignant tumor compared to another, excluding cases of direct extension, collision without true metastasis, or pre-existing lymphatic involvement. Herein, we present a rare case of a lung collision tumor involving large cell lung carcinoma (LCLC) and metastasis from colorectal signet ring cell carcinoma (SRCC).