Chen took over as CEO a month ago after BlackBerry abandoned a plan to sell itself.
"Our 'for sale' sign has been taken down and we are here to stay," he wrote in the letter that was addressed to "valued enterprise customers and partners". The company provided the letter to Reuters.
"In short, reports of our death are greatly exaggerated."
BlackBerry, once the market leader in on-the-go email, has suffered a drastic loss of market share to Apple Inc's iPhone and devices powered by Google Inc's Android software. Its new smartphones have so far failed to win back customers, as Chen acknowledged in the letter.
"We know that BlackBerry devices are not for everyone. That's OK," he said.
BlackBerry has laid off thousands of workers over the last two years and in September it said it would shed more than a third of its global workforce and refocus itself on the enterprise market - large business and government clients - that vaulted it to prominence in the 1990s.
"We're going back to our heritage and roots, delivering enterprise-grade, end-to-end mobile solutions," said Chen, a turnaround artist with software maker Sybase in the late 1990s.
"The investments you've made in BlackBerry infrastructure and solutions are secure. I will keep the lines of communication open as we navigate through this transition."
BlackBerry shares were up 5 Canadian cents at C$6.76 on the Toronto Stock Exchange on Monday, just above the record low of C$6.25 hit two weeks ago, and at just a fraction of the stock's record high of C$150.30, hit in 2008, when the company dominated the smartphone market.
Rather than sell itself, BlackBerry said early last month it would raise $1 billion in a convertible notes offering led by Fairfax Financial Holdings Ltd , its largest shareholder.