Net Interest Income. Net interest income increased $50.3 million, or 32%, to $205.3 million for the nine months ended September 30, 2021, compared with the nine months ended September 30, 2020. The increase was primarily due to a $1.7 billion increase in our average interest earning assets and a 31 basis point increase in our interest rate spread, to 2.76%, for the nine months ended September 30, 2021 from 2.45% for the nine months ended September 30, 2020.
Our net interest margin increased 26 basis points, to 2.82%, for the nine months ended September 30, 2021 from 2.56% for the nine months ended September 30, 2020. The increase in net interest margin reflected higher average loan balances and lower funding costs that outpaced the lower overall market interest rates on loans compared to the nine months ended September 30, 2020.
Interest Income. Interest income increased $24.4 million, or 12%, to $229.3 million for the nine months ended September 30, 2021, compared with the nine months ended September 30, 2020. This increase was primarily attributable to a $27.3 million increase in interest on loans, which was partially offset by a decrease of other interest-bearing assets and securities available for sale.
Interest income for loans and loans held for sale increased $27.3 million compared to the nine months ended September 30, 2020. The average balance of loans, including loans held for sale, during the nine months ended September 30, 2021 increased $1.7 billion, or 26%, to $8.3 billion from $6.6 billion for the nine months ended September 30, 2020, reflecting significant increases in loan volume. The average yield on loans decreased 34 basis points, to 3.48%, for the nine months ended September 30, 2021, compared to 3.82% for the nine months ended September 30, 2020, due to lower overall interest rates in the economy period to period.
Interest income for interest-bearing deposits and other assets decreased by $2.5 million compared to the nine months ended September 30, 2020. The average balance of interest-earning deposits and other assets decreased $118.6 million, or 15%, to $659.6 million for the nine months ended September 30, 2021 from $778.3 million for the nine months ended September 30, 2020, and the average yield decreased 39 basis points, to 0.31%, for the nine months ended September 30, 2021, compared to 0.70% for the nine months ended September 30, 2020.
Interest income for taxable available for sale securities decreased by $423,000 compared to the nine months ended September 30, 2020. The average balance of taxable available for sale securities increased $7.1 million, or 3%, to $287.3 million for the nine months ended September 30, 2021, compared to $280.2 million for the nine months ended September 30, 2020, while the average yield decreased 23 basis points to 1.07% for the nine months ended September 30, 2021, compared to 1.30% for the nine months ended September 30, 2020.
Interest Expense. Total interest expense decreased $25.9 million, or 52%, to $24.1 million for the nine months ended September 30, 2021, compared with the nine months ended September 30, 2020.
Interest expense on total deposits decreased $25.4 million, or 56%, to $19.8 million for the nine months ended September 30, 2021 compared to the nine months ended September 30, 2020. The decrease was attributable to lower volume and rates of brokered certificates of deposits and lower volume of interest-bearing checking at lower rates.
Interest expenses for certificates of deposit decreased $18.8 million compared to the nine months ended September 30, 2020. The average balance of certificates of deposits of $507.3 million for the nine months ended September 30, 2021 decreased $1.6 billion, or 76%, compared to the nine months ended September 30, 2020. The average yield of certificates of deposits was 0.85% for the nine months ended September 30, 2021, which was a 55 basis point decrease compared to 1.40% for the nine months ended September 30, 2020.
Interest expense for interest-bearing checking deposits decreased $6.5 million compared to the nine months ended September 30, 2020. The decrease was attributable to 37 basis point decrease in the average cost of interest-bearing checking deposits, to 0.12% for the nine months ended September 30, 2021 from 0.49% for the same period in 2020. Offsetting the lower average cost was a $1.8 billion, or 63%, increase in the average balance of interest-bearing checking deposits, which reached $4.7 billion for the nine months ended September 30, 2021.