In this modern era, a good product is not enough to please a customer. As time goes by, competition among companies as well as marketers are getting more fierce and intense. Companies have to think of ways to promote their products in efficient and effective ways. Promotional mix is more than just advertising or to sell a particular product or service. There are five indicators of promotional mix such as advertising, sales promotion, public relations, direct marketing, and personal selling. Customer’s buying decision is quite a complex variable to be studied since it is extremely subjective. There are also five indicators to customer buying decision such as need/problem recognition, information search, alternatives evaluation, purchase decision, and post-purchase decision. The population for this skripsi is going to be everyone who joins the event of COCOWORK@Clapham Medan and the writer is going to utilize the purposive sampling method. This skripsi is going to be a quantitative research as the writer is going to utilize descriptive statistic method (mean, median, and mode), data quality testing (validity, reliability, normality, linearity, coefficient of correlation, determination, and linear regression test), and hypothesis testing. Promotional mix does have an effect on customer buying decision at COCOWORK@Clapham Medan as proven through the result of the tcount which is higher than ttable, therefore Ha is accepted and Ho is rejected. The effect of promotional mix towards customer buying decision at COCOWORK@Clapham Medan is significantly strong as proven through the result of coefficient of correlation between the two variables and determination test