Adjective or past tense?

‘Surprendre’ is an irregular -re verb (prendre, apprendre reprendre etc), the past participle is simply “surpris”, with ‘surprise’ being the adjectival form with feminine gender agreement. 

‘Surprise’ is either a (feminine) noun or feminine adjectival form in French. There is no verb ‘surpriser ’.

https://www.lawlessfrench.com/verb-conjugations/surprendre/

Conjugate prendre and derivatives (+avoir) in the compound past in French (Le Passé Composé)

The infinitive of the verb "to surprise" is surprendre. And like the verb prendre itself, it forms the past participle as surpris. There's also a noun "the surprise", which is la surprise.

Thank you, most helpful.