लेखनी, कलम या पेन वह वस्तु है जिससे कागज पर स्याही द्वारा लिखा जाता है। कलम से बहुत से अन्य चीजों पर भी लिखा जाता है। प्राचीन काल से लेकर आजतक अनेक प्रकार की लेखनियाँ प्रयोग की जातीं हैं जैसे नरकट की कलम, पंख से बनी कलम, फ़ाउंटेन पेन, बॉल-पॉइंट पेन आदि।

A pen is a common writing instrument that applies ink to a surface, usually paper, for writing or drawing. Early pens such as reed pens, quill pens, dip pens and ruling pens held a small amount of ink on a nib or in a small void or cavity that had to be periodically recharged by dipping the tip of the pen into an inkwell. Today, such pens find only a small number of specialized uses, such as in illustration and calligraphy. Reed pens, quill pens and dip pens, which were used for writing, have been replaced by ballpoint pens, rollerball pens, fountain pens and felt or ceramic tip pens. Ruling pens, which were used for technical drawing and cartography, have been replaced by technical pens such as the Rapidograph. All of these modern pens contain internal ink reservoirs, such that they do not need to be dipped in ink while writing.