Advertiser Programmatic revenue grew $13.9 million, or 49%, in the three months ended September 30, 2021 as compared to the three months ended September 30, 2020, driven primarily by continued adoption of our premium-priced Authentic Brand Suitability solution. Advertiser Programmatic revenue grew $37.7 million, or 50%, in the nine months ended September 30, 2021 as compared to the nine months ended September 30, 2020, driven primarily by continued adoption of our premium-priced Authentic Brand Suitability solution. As previously disclosed in the Prospectus, we made a concession of $4.6 million to a Demand-Side-Platform partner which reduced Advertiser Programmatic revenue for amounts that were incorrectly billed by the partner and remitted to us in the period from January 2018 through December 2019. This concession was recorded in the third quarter of 2020.
Supply-Side revenue grew $1.7 million, or 32%, in the three months ended September 30, 2021 as compared to the three months ended September 30, 2020, primarily driven by increased uptake of our solutions from our platform and publisher customers. Supply-Side revenue grew $4.5 million, or 28%, in the nine months ended September 30, 2021 as compared to the nine months ended September 30, 2020, primarily driven by increased uptake of our solutions from our platform and publisher customers.
Cost of Revenue (exclusive of depreciation and amortization shown below)
Cost of revenue increased by $4.4 million, or 49%, from $9.0 million in the three months ended September 30, 2020 to $13.4 million in the three months ended September 30, 2021. The increase was primarily due to higher partner costs from revenue-sharing arrangements with our Advertiser Programmatic partners, as well as higher software and other technology costs to support our increased volumes. Cost of revenue increased by $12.0 million, or 50%, from $24.0 million in the nine months ended September 30, 2020 to $35.9 million in the nine months ended September 30, 2021. The increase was primarily due to higher partner costs from revenue-sharing arrangements with our Advertiser Programmatic partners, as well as higher software and other technology costs to support our increased volumes.
Product Development Expenses
Product development expenses increased by $3.3 million, or 25%, from $13.1 million in the three months ended September 30, 2020 to $16.4 million in the three months ended September 30, 2021. The increase was primarily due to an increase in personnel costs of $1.7 million, which reflects our continued hiring of resources to support our product-development efforts, and $1.0 million of additional stock-based compensation expense. Product development expenses increased by $11.3 million, or 33%, from $34.3 million in the nine months ended September 30, 2020 to $45.7 million in the nine months ended September 30, 2021. The increase was primarily due to an increase in personnel costs of $8.6 million, which reflects our continued hiring of resources to support our product-development efforts, in addition to $1.5 million of stock-based compensation expenses, $0.5 million of higher office expenses and $0.6 million of higher software expenses.
Sales, Marketing and Customer Support Expenses
Sales, marketing and customer support expenses increased by $2.8 million, or 17%, from $16.7 million in the three months ended September 30, 2020 to $19.5 million in the three months ended September 30, 2021. The increase was primarily due to an increase in personnel costs, including sales commissions, of $1.5 million to support our sales efforts, grow market presence in international markets, drive continued expansion with our existing customers, and support existing and new customers in addition to $1.1 million of higher stock-based compensation expense. Sales, marketing and customer support expenses increased by $12.8 million, or 30%, from $41.9 million in the nine months ended September 30, 2020 to $54.7 million in the nine months ended September 30, 2021. The increase was primarily due to an increase in personnel costs, including sales commissions, of $11.2 million to support our sales efforts, grow market presence in international markets, drive continued expansion with our existing customers, and support existing and new customers, in addition to increased stock-based compensation expenses of $2.9 million.