A substantial outstanding challenge in diagnostics and disease monitoring is an ability to rapidly and conveniently assay for protein biomarkers within complex biological media. Label-free electroanalytical methods present, arguably, the most promising and scalable means of achieving this but, as with all label-free assays, can struggle with response selectivity issues that arise from nonspecific surface interactions. Impedimetric methods are ultrasensitive and have been applied to the quantification of a wide range of proteins but have not previously been utilized in a multiplexed format capable of operation in complex analytical fluid. Herein, we present the use of thermally cross-linked poly(ethylene glycol) (PEG) polymer sensory array interfaces in the ultrasensitive quantification of two protein markers, insulin and C-reactive protein (CRP). This was achieved with detection limits of 171 ± 19 fM and 150 ± 10 pM, respectively. Significantly, the arrays not only enable the simultaneous, fast, nonamplified, and label-free detection of both markers without reagent addition but do so with little cross talk, even in human serum. A blind analysis of 17 real patient samples generated results in excellent agreement with those obtained through a clinically approved chemiluminescence assay.