sales during the nine months ended September 30, 2022 is due in part to the early 2021 digestion of equipment at key accounts and better supply availability, which enabled us to partially fulfill product shipments against higher customer demand.
Sales in the Telecom and Networking market increased $12.9 million, or 43.6%, to $42.5 million for the three months ended September 30, 2022 and $21.0 million, or 22.1%, to $116.0 million for the nine months ended September 30, 2022 as compared to the same periods in the prior year. The increase in sales was primarily due to increased demand in the Telecom and Networking market and improved material availability.
GROSS PROFIT
For the three months ended September 30, 2022, gross profit increased $71.2 million to $191.2 million, or 37.0% of revenue, as compared to $120.0 million, or 34.7% of revenue, in the same period in the prior year. For the nine months ended September 30, 2022, gross profit increased $105.1 million to $497.7 million, or 36.7% of revenue, as compared to $392.6 million, or 37.1% of revenue, in the same period in the prior year.
The increase in gross profit as a percent of revenue for the three months ended September 30, 2022 is largely due to operational leverage on sales growth. The decrease in gross profit as a percent of revenue for the nine months ended September 30, 2022 is largely related to higher material and freight costs, productivity inefficiencies, and other supply chain related costs. These factors began to affect Advanced Energy primarily in the second quarter of 2021.
OPERATING EXPENSE
Research and Development
We perform R&D of products to develop new or emerging applications, technological advances to provide higher performance, lower cost, or other attributes that we may expect to advance our customers’ products. We believe that continued development of technological applications, as well as enhancements to existing products and related software to support customer requirements, are critical for us to compete in the markets we serve. Accordingly, we devote significant personnel and financial resources to the development of new products and the enhancement of existing products, and we expect these investments to continue.
Research and development expenses increased $9.2 million to $49.8 million for the three months ended September 30, 2022 and increased $20.5 million to $141.4 million for the nine months ended September 30, 2022 as compared to the same periods in the prior year. The increase in research and development expense is related to the acquisitions of SL Power and TEGAM, increased headcount and associated costs, outside technical services, engineering materials and higher variable compensation related costs, as we invest in new programs to maintain and increase our technological leadership and provide solutions to our customers’ evolving needs.
Selling, General and Administrative
Our selling expenses support domestic and international sales and marketing activities that include personnel, trade shows, advertising, third-party sales representative commissions, and other selling and marketing activities. Our general and administrative expenses support our worldwide corporate, legal, tax, financial, governance, administrative, information systems, and human resource functions in addition to our general management, including acquisition-related activities.
Selling, general and administrative ("SG&A") expenses increased $8.3 million to $56.7 million for the three months ended September 30, 2022 and increased $17.8 million to $161.1 million for the nine months ended September 30, 2022 as compared to the same periods in the prior year. The increase in SG&A is principally related to acquisition related activities, sales commissions driven by higher revenue, an increase in headcount, and an increase in variable compensation.