The case against Adrian Jones, who was a senior vice-president in HP's server, storage and networking division in Asia, shows the growing rancor between the Silicon Valley technology titans.
HP's and Oracle's decades-long partnership is being strained by growing competition between the companies.
The company said Jones resigned Feb 16, before he was about to be fired for allegedly violating HP's standards.
He is accused in the lawsuit of failing to disclose a "close personal relationship" with a subordinate, and of submitting thousands of dollars in expenses for visiting the subordinate that didn't have a legitimate business purpose.
HP alleged Jones downloaded hundreds of files and thousands of e-mail messages detailing HP's secrets before he quit.
The company, based in Palo Alto, said the lawsuit "seeks to prevent Jones from using the theft of confidential HP documents to place HP in an unfair competitive disadvantage."
A phone number for Jones could not immediately be located.
The HP-Oracle rivalry kicked off with Oracle's US$7.3bil acquisition last year of Sun Microsystems, a server computer seller that competes with HP.
It ratcheted up with Oracle CEO Larry Ellison's ridiculing of HP's board in the fall for firing his friend and HP CEO Mark Hurd over inaccurate expense reports.
The rivalry continued with Oracle targeting Hurd's replacement, Leo Apotheker, in a separate court case involving the theft of trade secrets and his former employer, SAP AG.