There is a growing demand of comfortable and unobtrusive wearable systems for monitoring a variety of physiological parameters and in particular the respiratory frequency (<inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula>). The most popular techniques for <inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula> monitoring cannot be used in several clinical applications and in unstructured environment. These issues have fostered a dramatic growth of interest for wearable systems devoted to monitor <inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula>. In this arena, fiber Bragg grating (FBG) sensors have gained due to a variety of benefits. In this work, we designed and fabricated an FBG-based wearable device for <inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula> monitoring from the nasal airflow. The proposed design does not require a mask to improve the comfortability and acceptability of the system. The sensing element was functionalized by a hygroscopic coating material to make the FBG sensitive to relative humidity changes. This feature allows calculating <inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula> starting from the discrimination between the inspiration and expiration phases. A pilot study on 6 volunteers was designed to assess the system during three different breathing stages (i.e., slow, normal and fast breathing). Results showed high performance of the proposed wearable device in detecting mean and breath-by-breath <inline-formula> <tex-math notation="LaTeX">${f}_{R}$ </tex-math></inline-formula> values (i.e., mean percentage errors ≤ 2.29 % and bias ≤ 0.31 breaths per minute) during slow breathing, normal breathing, and fast breathing.