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• All fiber-optic sensor based on a Balloon-like interferometer for pH measurement was demonstrated. • The Mach-Zehnder interferometer was fabricated by bending standard single-mode fiber into a Balloon shape. • In addition to the high sensitivity, the proposed sensor exhibits a fast response time with excellent stability. In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability. [ABSTRACT FROM AUTHOR]