Large-scale,　rapid,　and　inexpensive　serological　diagnoses　of　severe　acute　respiratory　syndrome-coronavirus-2　(SARS-CoV-2)　are　of　great　interest　in　reducing　virus　transmission　at　the　population　level;　however,　their　development　is　greatly　plagued　by　the　lack　of　available　point-of-care　methods,　leading　to　low　detection　efficiency.　Herein,　an　ultrasensitive　smartphone-based　electrochemical　immunoassay　is　reported　for　rapid　(less　than　5　min),　low-cost,　easy-to-implement　detection　of　the　SARS-CoV-2　nucleocapsid　protein　(SARS-CoV-2　N　protein).　Specifically,　the　electrochemical　immunoassay　was　fabricated　on　a　screen-printed　carbon　electrode　coated　with　electrodeposited　gold　nanoparticles,　followed　by　incubation　of　anti-N　antibody　(Ab)　and　bovine　serum　albumin　as　the　working　electrode.　Accompanied　by　the　antigen-antibody　reaction　between　the　SARS-CoV-2　N　protein　and　the　Ab,　the　electron　transfer　between　the　electroactive　species　[Fe(CN)6]3-/4-　and　the　electrode　surface　is　disturbed,　resulting　in　reduced　square-wave　voltammetry　currents　at　0.075　V　versus　the　Ag/AgCl　reference　electrode.　The　proposed　immunoassay　provided　a　good　linear　range　with　SARS-CoV-2　N　protein　concentrations　within　the　scope　of　0.01-1000　ng/mL　(R2　=　0.9992)　and　the　limit　of　detection　down　to　2.6　pg/mL.　Moreover,　the　detection　data　are　wirelessly　transmitted　to　the　interface　of　the　smartphone,　and　the　corresponding　SARS-CoV-2　N　protein　concentration　value　is　calculated　and　displayed.　Therefore,　the　proposed　portable　detection　mode　offers　great　potential　for　self-differential　diagnosis　of　residents,　which　will　greatly　facilitate　the　effective　control　and　large-scale　screening　of　virus　transmission　in　resource-limited　areas.　©　2022　American　Chemical　Society.