A variable-rate mortgage, adjustable-rate mortgage (ARM), or tracker mortgage is a mortgage loan with the interest rate on the note periodically adjusted based on an index which reflects the cost to the lender of borrowing on the credit markets. The loan may be offered at the lender's standard variable rate/base rate. There may be a direct and legally defined link to the underlying index, but where the lender offers no specific link to the underlying market or index, the rate can be changed at the lender's discretion. The term "variable-rate mortgage" is most common outside the United States, whilst in the United States, "adjustable-rate mortgage" is most common, and implies a mortgage regulated by the Federal government, with caps on charges. In many countries, adjustable rate mortgages are the norm, and in such places, may simply be referred to as mortgages.

Among the most common indices are the rates on 1-year constant-maturity Treasury (CMT) securities, the cost of funds index (COFI), and the London Interbank Offered Rate (LIBOR). A few lenders use their own cost of funds as an index, rather than using other indices. This is done to ensure a steady margin for the lender, whose own cost of funding will usually be related to the index. Consequently, payments made by the borrower may change over time with the changing interest rate (alternatively, the term of the loan may change). This is distinct from the graduated payment mortgage, which offers changing payment amounts but a fixed interest rate. Other forms of mortgage loan include the interest-only mortgage, the fixed-rate mortgage, the negative amortization mortgage, and the balloon payment mortgage.

Adjustable rates transfer part of the interest rate risk from the lender to the borrower. They can be used where unpredictable interest rates make fixed rate loans difficult to obtain. The borrower benefits if the interest rate falls but loses if the interest rate increases. The borrower also benefits from reduced margins to the underlying cost of borrowing compared to fixed or capped rate mortgages.

In contrast to fixed-rate mortgages, adjustable-rate mortgages are unaffected by inflation risk, but they are exposed to the risk that real interest rates will change. Adjustable-rate mortgages usually charge lower interest rates than those with fixed rates. According to scholars, "borrowers should generally prefer adjustable-rate over fixed-rate mortgages, unless interest rates are low."