This work proposes a battery-less Near-Field Communication (NFC) milk conductivity sensor for the early-stage detection of bovine mastitis. A proof-of-concept prototype is designed, which is composed of a loop antenna, an NFC microchip, a microcontroller, a custom conductivity sensor and a temperature sensor. A mobile phone is used as a reader, which additionally provides energy to the sensor to power all the electronic components. Since milk conductivity is temperature-dependent, it is also important to measure the temperature. The prototype has a precision better than 2%, which is similar to that of commercial hand-held conductivity meters. Measurements of the conductivity of cows’ milk are made to establish limits that allow healthy cows to be discriminated from those suffering from clinical or subclinical mastitis. To this end, the obtained results are analyzed through ANOVA. The reliability of the method can be improved when milk samples from each quarter are used in the classification. This low-cost device has great potential to be used as on-farm screening tool for the quick identification of bovine mastitis.