A microwave applicator is presented that realizes the theranostic approach by providing two operation modes to detect and treat tumor cells, exemplary in the liver. The tool is based on a planar sensor structure with Split Ring Resonators (SRR) as key elements. For the detection, changes in dielectric properties caused by tissue abnormalities are evaluated by analyzing shifts in the resonance frequency of the SRR. For the treatment mode, the input power is amplified to the specific resonance frequency and microwave ablation is performed. The device is designed to fit into a needle like operation tool for minimal invasive therapies. From this requirement, challenges concerning size limitation, packaging, and bio compatibility can be derived. Therefore, novel shapes of the SRR and substrate materials are investigated by full wave simulations and measurements. Material models and phantoms mimicking dielectric properties of healthy and malignant tissue are introduced. Clear resonance shifts of 100 MHz are detected for loading the sensor with corresponding phantoms. In order to examine the treatment mode characteristics, thermal simulations and measurements exhibit a significant temperature increase up to 62 °C for an input power of 500 mW.