This paper presents analytical and experimental studies of a modified semiactive hydraulic damper operating in periodic working regimes. This work was completed as a part of the Rotor Embedded Actuator Control Technology project sponsored by the Technology Strategy Board in the United Kingdom. The damper tested is based on modifications to an industrially employed helicopter damper. The work presented covers the relevant aspects of the model development, damper modification, test planning, and analysis, as well as a model-simulation correlation study. The model of the damper directly reflects the actual hydraulic modification, enabling a semiactive mode of operation. Pressure-flow representations used in the damper modeling are based on a novel testing methodology using triangular piston excitation waveforms. A specific test structure based on varying the relative phase difference between two harmonic input signals is used to assess the damper properties in periodic working regimes. The test configuration with a base harmonic piston excitation combined with a harmonic modulation at a frequency 3 times higher than the base frequency leads to significant changes in the second and fourth harmonic components of the resulting periodic damper forces. A successful model-simulation correlation study suggests that a simple one-state dynamic model of this damper features good predictive properties, equally applicable in the extended simulation contexts.