The company said Senk "plans to pursue another opportunity" but will remain "for a period of time to assist management with the transition." Senk also resigned from the board.
A message left for Senk at his office was not immediately returned. Nor was a message left with chief financial officer Eric Artz.
Hayne founded Urban Outfitters in 1970 and remained in charge until the position of CEO was created and Senk filled it more than four years ago.
"Mr. Hayne has been instrumental in the company's historical success, and we are extremely fortunate to have an executive of Dick's tenure and caliber in place and prepared to lead the company's continued growth," the board said in a statement.
Senk's resignation, which the company said was tendered Monday and effective immediately, was made public after the stock market closed at 4 p.m. - two days before executives were expected to appear Thursday at a conference for institutional investors in Miami, the 14th annual ICR XChange.
The news set off shock waves among investment analysts and investors, who could only speculate why Hayne's trusted successor was leaving - and why now, since executives had assured investors that business would improve soon.
As recently as November, Senk had announced a sweeping restructuring of the management ranks that included the creation of an executive to oversee the troubled Anthropologie brand.
In addition to Anthropologie and Urban Outfitters, the corporation includes retail brands Free People, Terrain, and BHLDN, a bridal line pronounced "beholden." A designer-apparel line, Leifsdottir, launched under Senk, was discontinued as he sought to restructure operations.
"They were looking forward to the spring of 2012; the new spring lines will be arriving in stores this February," said investment analyst Margaret Whitfield, of Sterne Agee. "Whether or not that's enough to turn the tide remains to be seen."
Whitfield, who with other analysts is in Florida for the conference, said she had not been expecting the announcement but declined to elaborate until learning more.
Her surprise was shared by others, gauged partly by the beating the company's shares took following the news. One interpretation of the stock sell-off was that investors feared the change at the top might be a harbinger of poorer than expected profits for Urban in the just-completed holiday shopping season.
Shares ended the trading day at $29.41 on the Nasdaq, but they plummeted more than 15 percent to $24.86 in the hour after the announcement went public. (Shares were trading at $23.95 at midmorning Wednesday.)
It was unclear what sort of severance package Senk received; no details were disclosed. In 2010, his total compensation was $2,513,118, according to Equilar Inc.
Urban Outfitters' stock has languished since March, when officials disclosed lackluster sales over the 2010 holidays. Shares climbed above $38 in February before tumbling 17 percent in March.
What followed were successive quarters of more bad news that women's apparel at Anthropologie and Urban Outfitters was not selling, forcing markdowns to clear racks.
In April, the company lost its global president of the Urban Outfitters brand when Steve Murray resigned after only a year. In May, Senk told investors he would personally oversee operations at Anthropologie.
In August, the company reported another profitable quarter, but by a smaller margin than the previous year.
November's management-lineup revamping preceded a holiday season that saw Anthropologie and Urban Outfitters merchandise heavily marked down, both on Black Friday and into January, said analyst Whitfield.
Last week, the company said that its sales were up 11 percent for November and December but that officials had discounted merchandise more than they had planned, a source of speculation that quarterly profits might be lower than expected when financial results for the fourth quarter are disclosed in the weeks ahead.
Contact staff writer Maria Panaritis at 215-854-2431, firstname.lastname@example.org, or @panaritism on Twitter.