Transferring single-crystalline (SC) membranes to flexible substrates has been increasingly studied, enabling emerging functionality and enhanced performance of various devices. A commonly used support-assisted transfer process inevitably leaves dirty residue on material surfaces, limiting the further development of surface-related applications. Here, we scale down the thickness of flexible SC SrRuO3 (SRO) membranes to 15 nm with a clean surface area of 2.5 x 2.5 mm(2). This is accomplished by making the polyethylene terephthalate (PET) substrate surface hydrophilic via oxygen plasma treatment, thereby reducing the surface tension. The ultrathin, clean, wide, and flexible SC SRO membranes guarantee a high transmittance of up to 60%, a low resistivity of 10(-4)-10(-3) Omega cm at room temperature, and band ferromagnetism below 150 K with a high magnetic moment of similar to 0.5 mu(B)/Ru at 10 K. The SC-level properties of our SRO membranes imply their potential use in state-of-the-art platforms for next-generation electronics and energy devices.