Minneapolis-based investment banker Piper Jaffray Cos. has agreed to purchase Seattle-Northwest Securities Corp. in a transaction valued at $21 million.

Piper also runs a public finance office in Seattle, but said in a statement late Wednesday that it has minimal "geographic and business overlap with Seattle-Northwest" and that the merger will strengthen Piper's business.

"This is a great combination for both firms," Piper CEO Andrew Duff said in a prepared statement. "A key part of our firm's strategy is investing in our public finance business, and this represents a significant step forward in developing a national franchise for our business."

CEO Karl Leaverton of Seattle-Northwest said "our employees and clients will benefit from Piper Jaffray's specialized products, deep inventory and strong capital base."

Seattle-Northwest specializes in underwriting municipal securities, and its sales and trading division distributes bonds nationwide.

Piper said that the tangible book value of Seattle-Northwest upon closing is estimated to be $13 million. Piper is paying a significant premium to buy Seattle-Northwest. The agreement, pending approval by Seattle-Northwest's shareholders and regulators, should close in the second half of 2013.

Neal St. Anthony • 612-673-7206