Measurement of antibody and antibody fusion protein concentration is vital for process development and manufacturing. Continuous, in‐line monitoring of antibody concentration could be useful in a variety of applications, such as controlling the loading of protein A columns to prevent breakthrough, monitoring bioreactor titer, and detecting leaks past ultrafiltration/diafiltration membranes. Molecule‐specific monitoring techniques are advantageous for antibody detection in cell culture fluid in the presence of complex process impurities. Here we report a continuous in‐line, real‐time IgG monitoring platform using a fiber‐optic biosensor with a replaceable sensor tip covalently functionalized with a fluor‐labeled protein consisting of a pentamer of the Z domain (a more stable form of the B domain) of protein A. The sensor demonstrates concentration‐dependent fluorescence enhancement in the presence of human IgG (0.01–0.75 g/L), with consistent signals during five runs each with 1 and 0.1 g/L IgG, and maintains its specificity in the presence of Chinese hamster ovary (CHO) cell culture fluid. A 5% breakthrough of a typical 10 g/L load would be detected in less than 20 s in a flowing stream emerging from a protein A column without prior sample preparation. This sensor platform may be suitable for monitoring IgG and fragment, crystallizable (Fc) fusion proteins in diverse upstream and downstream bioprocess applications.