is a hypothetical structure in Japanese consisting of the adverb そう ‘like that’, and the ば conjunctive form of the う-Verb 言（い）う ‘to say’. Common translations include ‘speaking of which’, ‘by the way’, and ‘come to think of it’. These translations are actually very close to the literal translation of ‘if saying that’ in that they all refer back to the previous statement as being a trigger for bringing up a topic that the speaker wants to discuss.
will primarily appear at the beginning of sentences, before the speaker will then bring up the topic that they were reminded of.
By the way, I finished reading the book I borrowed from you the other day! It was super interesting!
Speaking of which, did you transfer the rent to the landlord for me?
Speaking of which, I met your friend at the station. She told me to tell you she said hi.
Caution - Due to そういえば
refering to something that has already been brought up over the course of a conversation, it will not be used to move on to completely unrelated topics in the same way that phrases like ‘by the way’ are used in English.
Tanaka-san: Let’s go to the batting cage next time!
Takahashi-san: Speaking of which, did you hear that Fujita-san and Ito-kun has started dating? (Unnatural Japanese)
Fun-fact - そういえば is very similar in the way it is used to といえば, with そう simply replacing the direct quotation of と. Many textbooks will list both as a single grammar point, despite そういえば itself basically being a set phrase.