Single-photon avalanche diodes (SPADs) are widely used for practical applications requiring single-photon detection. The readout circuit, or quenching electronics, plays an important role for the operations of SPADs. Sine wave gating (SWG) is one of the key techniques for synchronous single-photon detection that can easily operate SPADs with a gating frequency as high as GHz level. Here we present a monolithic readout circuit for 1.25 GHz SWG SPADs. The monolithic chip, including a low-noise amplifier and two low-pass filters inside, is designed for weak avalanche extraction in the SWG scheme and fabricated using the technology of low temperature co-fired ceramic with a size of 15 mm × 15 mm. We then apply the monolithic chip into an InGaAs/InP single-photon detector (SPD). After the characterization both on the monolithic chip and the InGaAs/InP SPD, the functionality of the monolithic readout circuit is effectively verified. Implementing the monolithic integration of readout circuit is a key step towards developing miniaturized InGaAs/InP SPDs.