Breast cancer remains a major cause of cancer-related mortality in women, and its substantial molecular heterogeneity highlights the need for methods capable of characterizing tumor cells at the cellular level. CD44, a transmembrane glycoprotein associated with breast cancer progression, metastasis, and cancer stem-like phenotypes, is a promising biomarker for selective recognition of aggressive, basal-like cell populations. However, conventional approaches to evaluating CD44 expression including immunohistochemistry, immunofluorescence, and flow cytometry require labeling and multi-step sample preparation. In this work, a gold-coated, polydimethylsiloxane (PDMS)-based extrinsic semi-distributed interferometric (SDI) optical fiber biosensor functionalized with anti-CD44 antibodies was developed for label-free, selective detection of CD44-expressing HCC1806 breast cancer cells. The functionalized sensor exhibited a concentration-dependent response with a limit of detection (LoD) of 2.51 cells/mL, while the antibody-free control sensor exposed to HCC1806 cells and the anti-CD44-functionalized sensor exposed to CD44-low HEK293 cells showed only minimal responses, supporting selective detection under the tested conditions. These results support the potential of the antibody-functionalized ESDI platform for label-free detection of CD44-expressing cells, providing a groundwork for its validation in clinical samples as a step toward tumor-cell diagnostics.