Highly concentrated micellar casein concentrate (HC-MCC), a potential ingredient for protein fortification, containing ~20% casein with most whey proteins removed by microfiltration, diafiltration, and vacuum evaporation of skim milk. Our objective was to determine the conditions needed to obtain complete solubility of thawed frozen HC-MCC in water, and to understand its gelation upon cooling. Dispersibility (ability to pass through a 250-µm mesh sieve), suspendability (percentage of protein not sedimented at 80 × g within 5 min), and solubility (percentage of protein not sedimented at 20,000 × g within 5 min) were measured at various mixing conditions. Gelation upon cooling from 50°C to 5°C was monitored based on storage (G′) and loss (G″) modulus and gelled HC-MCC examined using transmission electron microscopy. Thawed HC-MCC was added to water (3% of protein) using high shear (7,500 rpm) for 1 min or low shear (800 rpm) for 30 min at 4, 12, 20, or 50°C and at pH 6.4 to 7.2. The HC-MCC completely dispersed at 50°C, or at ≤ 20°C followed by overnight hydration at 4°C. Suspendability at 50°C was ~90%, while mixing at ≤ 20°C followed by overnight hydration yielded only ~57%. Solubility followed a similar trend with ~83% at 50°C and only ~29% at ≤ 20°C. Mixing HC-MCC in 60 mM trisodium citrate increased dispersibility to 99%, and both suspendability and solubility to 81% at 20°C. Cold gelling temperature, defined as temperature at which G′ = G″, was positively correlated (R² = 0.97) with protein level in HC-MCC. Gelation occurred at 38, 28 and 7°C with 23, 20, and 17% of protein, respectively. Gelation was reversible upon heating. In micrographs of gelled HC-MCC, casein micelles were observed to be within the normal range and close packed together with only ~20 to 50 nm space between them. We propose that cold gelation of HC-MCC occurs when the kinetic energy of the casein micelles is sufficiently reduced to inhibit their mobility in relation to adjacent casein micelles. Understanding solubilization of rehydrated frozen HC-MCC and its rheological property can help designing process systems for using it as a potential ingredient in liquid food.