This study examined the effects of different calcium salts—calcium carbonate, calcium hydroxide, and calcium chloride—on the softening of onion tissues during storage. Specifically, it focused on analyzing the enzymatic polygalacturonase (PG) and pectin methylesterase (PE) activities, calcium ion concentrations, tissue strength, hardness, and color changes in onions treated with various concentrations of these calcium salts. Calcium carbonate at a concentration of 70 mM was the most effective at inhibiting PG activity, reducing it by approximately 4.4 times compared to untreated onions after 60 days of storage. Calcium hydroxide also reduced the PG activity significantly, with the 90 mM treatment being the most effective, albeit less impactful than calcium carbonate. Calcium chloride at 20 mM lowered the PG activity by 2.8 times but was less effective than calcium carbonate. PE activity varied according to the type and concentration of the calcium salt used. Calcium chloride at 20 mM was the most effective in enhancing the PE activity. In terms of the calcium ion concentration, calcium chloride showed the highest levels of penetration, with the 60 mM treatment group reaching a peak concentration of 33.435 mM after 30 days of storage. The strength and hardness tests showed that calcium chloride consistently maintained the highest strength and hardness levels over the storage period, particularly at 40 mM and 90 mM. Regarding the color changes, the L*, a*, and b* values indicated that onions treated with calcium carbonate closely resembled the control group in terms of color stability, while those treated with calcium chloride retained better overall color throughout storage. Therefore, among calcium carbonate, calcium hydroxide, and calcium chloride, treatment with 20 mM calcium chloride resulted in the most effective inhibition of onion tissue softening.