HER2 amplification has been identified in 2% to 3% of all colorectal cancers (CRCs). Although the prognostic role of HER2 amplification in metastatic CRC (mCRC) is unclear, studies have highlighted it as a therapeutic target. In addition, several studies have shown that HER2 amplification is implicated in the resistance to EGFR-targeted therapies. Other studies have provided scientific evidence to support the use of HER2-directed therapies in HER2-amplified CRC; however, thus far this benefit has been limited to the RAS wild-type population. There is an ongoing clinical need to identify novel means of targeting HER2 amplifications in the rare settings of HER2-amplified, RAS-mutated CRC. This case report presents a 58-year-old man with HER2-amplified mCRC and a KRAS G12D mutation whose disease progressed on all standard cytotoxic therapies as well as dual HER2 targeting using trastuzumab and pertuzumab. He subsequently derived a clinical benefit with metastatic lung disease regression on trastuzumab emtansine (T-DM1). He eventually experienced disease progression in the liver after 6 every-3-week cycles. The patient's response and disease progression were associated with ongoing decline in the HER2 copy number on the circulating tumor DNA assay, suggesting that the mechanism of resistance was related to the loss of HER2 amplification or the emergence of non-HER2-amplified CRC clones. This represents the first report of clinical benefit with T-DM1 in KRAS-mutated HER2-amplified CRC.