Background and Objectives: One promising direction for developing environmentally friendly electromagnetic wave absorbers involves using structures that incorporate polar liquids, such as water. This is because water in the microwave range exhibits both a significant real part and a significant imaginary part of the complex permittivity. Such structures can be either continuous water layers or composite surfaces comprising individually arranged periodic droplets. Compared to more traditional materials, electromagnetic radiation absorbers based on composite water-containing structures offer several advantages, such as biocompatibility, availability, ease of fabrication, and optical transparency. One example of such a composite is cellulose paper acting as a matrix, filled with distilled water. Composite structures based on cellulose and its derivatives with inclusions of carbon nanotubes and silver nanowires are already known to be used as absorbers. Using an absorber made of such a composite material as an interface layer in a photonic crystal enables the observation of various resonance effects arising from the specific properties of the interface. Altering the electromagnetic characteristics of this interface layer leads to changes in the frequency and amplitude of the photonic Tamm resonance. Therefore, it is of interest to study the emergence of Tamm resonances within the band gap of a microwave photonic crystal that utilizes an electromagnetic energy absorber based on water-containing cellulose paper as the interface layer. Furthermore, this study aims to demonstrate interface control by varying the mass fractions of distilled water. Materials and Methods: The experimental structure was fabricated as a photonic crystal comprising alternating layers of two different dielectric materials inside a rectangular waveguide. A layer of cellulose paper containing distilled water was sandwiched between two Teflon films and placed immediately adjacent to the last layer of the photonic crystal. A vector network analyzer was used to measure the frequency characteristics in the 7–13 GHz frequency range. Numerical calculations for the layered structures were performed using the transfer matrix method. Results: As the thickness of the cellulose paper containing distilled water (with a mass fraction of more than 51%) increases, damped oscillations of the Tamm resonance frequencies have been observed within the first and second band gaps. Increasing the cellulose paper thickness has also led to damped oscillations of the reflectivity coefficients in these band gaps. For greater cellulose paper thicknesses, these values have been stabilized, becoming constant. The final values depend on both the mass fraction of water and the size of the air gap between the last layer of the photonic crystal and the cellulose paper layer. Conclusion: Adjusting the interface parameters by regulating the air gap between the photonic crystal and the cellulose layer allows for control over the Tamm resonance amplitude. For each fixed cellulose layer thickness with a specific mass fraction of distilled water, a particular air gap value must be selected to achieve the maximum Tamm resonance amplitude.