Here, we present chemically stable and instantly degradable (CSID) hydrogel immunospheres for the isolation of circulating tumor cells (CTCs) and circulating tumor exosomes (CTXs). The CSID hydrogels, which are prepared by the hybridization of alginate and poly(vinyl alcohol), show an equilibrium swelling ratio (ESR) of at pH 7, with a highly stable pH-responsive property. The present hybrid hydrogel is not easily disassociated in the biological buffers, thus being suitable for use in "liquid biopsy", requiring a multistep, long-term incubation process with biological samples. Also, it is gradually degraded by the action of chelating agents; effortless retrieval of the circulating markers has been achieved. Then, we modified the CSID hydrogel spheres with the anti-EpCAM antibody ("C-CSID ImmunoSpheres") and the anti-CD63 antibody ("E-CSID ImmunoSpheres") to isolate two promising circulating markers in liquid biopsy: CTCs and CTXs. The immunospheres' capabilities for marker isolation and retrieval were confirmed by a fluorescence image, where the spheres successfully isolate and effortlessly retrieve the target circulating markers. Lastly, we applied the CSID hydrogel immunospheres to five blood samples from colorectal cancer patients and retrieved average 10.8 ± 5.9 CTCs/mL and average 96.5 × 106 CTXs/mL. The present CSID hydrogel immunospheres represent a simple, versatile, and time-efficient assay platform for liquid biopsy in the practical setting, enabling us to gain a better understanding of disease-related circulating markers.