Capital Gains Inflation Relief Act of 2021 This bill allows the adjusted basis of certain assets (including any common stock in a C corporation, any digital asset, and tangible property used in a trade or business) to be indexed for inflation for the purpose of determining the gain or loss of a taxpayer (other than a corporation) who has held the asset for more than three years. The bill sets forth rules for applying the inflation adjustment to short sales; regulated investment companies; real estate investment trusts; other pass-through entities, including partnerships, S corporations, and common trust funds; dispositions between related persons; and improvements to property or contributions of capital. The Internal Revenue Service may disallow an adjustment if any person transfers cash, debt, or any other property to another person for the principal purpose of securing or increasing the adjustment allowed by this bill.