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Riboflavin (RF) is an important vitamin necessary for human living activities. It is a crucial component of the body’s critical enzymes, contributing to energy production, development, and metabolism. Low-cost RF sensors are one of the most essential research objectives for today’s development. In this work, the authors developed a highly sensitive optical platform for real-time detection of RF concentration, with the goal of developing and testing a novel RF sensor based on localized surface plasmon resonance (LSPR) with a tri-tapered-in-tapered (TTIT) seven-core fiber with multimode structure for fast and selective RF concentration measurement in solution. In this work, a TTIT fiber-based RF sensor was developed using a fusion splicer and tapered fiber optic fabrication techniques, in which gold nanoparticles (AuNPs), zinc oxide nanowires (ZnO-NWs), and molybdenum disulfide nanosheets (MoS2-NSs) were uniformly coated on the surface of the optical fiber to enhance the evanescent field in the sensing region, reducing optical signal loss and increasing the sensing area. To detect changes in RF concentration, the evanescent field can stimulate the LSPR of AuNPs immobilized on the probe surfaces. Furthermore, the sensor has great repeatability and stability, and the RF fiber-optic sensor developed in this experiment is an efficient, sensitive, and cost-effective mode of detection for rapid monitoring of RF levels in a wide range of practical applications. The sensor’s sensitivity was 2.14 nm/mM, with a limit of detection (LOD) of $86.86~\mu $ M. The success of the technique will encourage the development of RF detection technologies for food safety and clinical diagnosis.