must I take off my wristwatch before putting on tefillin shel yad?
ב"ה Shalom The Shulchan Aruch (אורח חיים סי' כז:ד ) says" One should not have any object separating between the tefillin and skin, whether it is the shel yad or shel rosh. To this the Rema adds: "This is referring to Tefillin(boxes) but in regard to the straps we need not be strict. The Mishna Berura there adds, that we should be stringent even in regard to the straps which are adjacent to the tying (of the Tefilllin box around your arm). From what have seen until here, we really don't have to be concerned in regard to a wristwatch since it's only under the straps and where the wristwatch is placed, is nowhere near the tying. Nonetheless, other Rabbis (פמ"ג מ"ז סק:ד, פסקי תשובות סי' כ"ד: ט) are of the opinion that it is proper that even the straps be totally on the skin. Therefore, you worded your question correctly. "Must I take off my wristwatch …",and the answer is: You don't have to, since it does not constitute a separation. However, it is still the proper thing to do. Even so, if the removing the wristwatch creates difficulty for you or discomfort , then you may leave on your wristwatch. All the best