This work presents design methods for first-order reflectionless lumped-element lowpass and bandpass filters. Each of the proposed filter structures contains an impedance matching circuit that can theoretically cancel out the reflected wave. We have derived the analytic design equations so that the reflectionless filters can be designed without using any sophisticated math techniques. In addition, by virtue of the design equations, the reflectionless lowpass and bandpass filters can be designed in a way to produce exactly the same transmitted power ratio as the conventional reflective filters. Both conventional reflective filters and proposed reflectionless filters have been fabricated and measured to verify the newly presented reflectionless filter design theory. The experimental results show that the proposed filter structures have excellent return loss performances in the stopbands as well as in the passbands. In addition, the measured transmitted power ratios of the reflectionless filters are not distinguishable of the reflective filters.